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Practice › LeetCode › LeetCode 34xx ›

LeetCode 3438 - Find Valid Pair of Adjacent Digits in String

LeetCode Problem 3438 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem gives us a string s containing only digits from '1' to '9' . We need to find the first adjacent pair of digits that satisfies two conditions. First, the two digits in the pair must be different. A pair like "22" or "55" is never valid. Second, each digit in the pair must appear...

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LeetCode 3675 - Minimum Operations to Transform String

LeetCode Problem 3675 Difficulty: 🟡 Medium Topics: String, Greedy Solution Problem Understanding We are given a string s consisting of lowercase English letters. An operation chooses a character value c and simultaneously replaces every occurrence of c in the current string with the next letter of the alphabet. The alphabet is circular, so 'z' becomes 'a' . The important detail is that an operation acts on a letter value, not...

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LeetCode 3615 - Longest Palindromic Path in Graph

LeetCode Problem 3615 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Bit Manipulation, Graph Theory, Bitmask Solution Problem Understanding We are given an undirected graph with n ≤ 14 vertices. Each vertex has a character label. We may choose any simple path in the graph, meaning a sequence of adjacent vertices in which no vertex is visited more than once. If the labels encountered along that path form a palindrome, the...

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LeetCode 3602 - Hexadecimal and Hexatrigesimal Conversion

LeetCode Problem 3602 Difficulty: 🟢 Easy Topics: Math, String Solution Problem Understanding The numbers that still fit in standard integer types; the algorithm scales logarithmically with digit count.

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LeetCode 3628 - Maximum Number of Subsequences After One Inserting

LeetCode Problem 3628 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Greedy, Prefix Sum Solution Problem Understanding We are given a string $s$ consisting of uppercase English letters. From this string, we are interested in counting subsequences equal to the pattern "LCT" , where a subsequence means indices $i < j < k$ such that $s[i] = \text{L}$, $s[j] = \text{C}$, and $s[k] = \text{T}$. We are allowed to insert at...

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LeetCode 3889 - Mirror Frequency Distance

LeetCode Problem 3889 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution LeetCode 3889 - Mirror Frequency Distance Problem Understanding We are given a string s containing only lowercase English letters ( a-z ) and digits ( 0-9 ). Each character has a corresponding mirror character: Letters are mirrored across the alphabet. a ↔ z b ↔ y c ↔ x and so on. Digits are mirrored across the digit...

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LeetCode 3884 - First Matching Character From Both Ends

LeetCode Problem 3884 Difficulty: 🟢 Easy Topics: Two Pointers, String Solution LeetCode 3884 - First Matching Character From Both Ends Problem Understanding This problem gives us a string s of length n consisting of lowercase English letters. For every index i , we compare two characters: The character at position i , which is s[i] The character at the mirrored position from the other end, which is s[n - i...

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LeetCode 3784 - Minimum Deletion Cost to Make All Characters Equal

LeetCode Problem 3784 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Enumeration Solution Problem Understanding The problem asks us to minimize the total cost of deleting characters from a string such that the resulting string contains only one unique character . We are given a string s of length n and an array cost of the same length, where cost[i] is the cost of deleting the character s[i] . The...

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LeetCode 3781 - Maximum Score After Binary Swaps

LeetCode Problem 3781 Difficulty: 🟡 Medium Topics: Array, String, Greedy, Heap (Priority Queue) Solution Before I write the full guide, I need one clarification: can you confirm the exact LeetCode 3781 problem details or provide the official title/link? “Maximum Score After Binary Swaps” does not match a problem I can reliably identify from memory, and I want to ensure the algorithm and proof are correct. In particular, I want to...

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LeetCode 3775 - Reverse Words With Same Vowel Count

LeetCode Problem 3775 Difficulty: 🟡 Medium Topics: Two Pointers, String, Simulation Solution Problem Understanding The problem requires processing a string s consisting of lowercase English words separated by single spaces. The first task is to count the number of vowels in the first word . Then, for each subsequent word in the string, if the word has the same number of vowels as the first word, it should be reversed....

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LeetCode 3864 - Minimum Cost to Partition a Binary String

LeetCode Problem 3864 Difficulty: 🔴 Hard Topics: String, Divide and Conquer, Prefix Sum Solution LeetCode 3864 - Minimum Cost to Partition a Binary String Problem Understanding We are given a binary string s , where each character represents whether an element is sensitive: '1' means the element is sensitive. '0' means the element is not sensitive. The entire string initially forms one segment. Every segment has a storage cost that...

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LeetCode 3773 - Maximum Number of Equal Length Runs

LeetCode Problem 3773 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to analyze a string s consisting of lowercase English letters and identify runs , which are contiguous sequences of identical characters that cannot be extended further. For example, in "aaabaaa" , the runs are "aaa" , "b" , and "aaa" . The goal is to select runs that all have the same...

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LeetCode 3863 - Minimum Operations to Sort a String

LeetCode Problem 3863 Difficulty: 🟡 Medium Topics: String Solution Problem Understanding This problem is asking us to determine the minimum number of substring sort operations needed to make a string s sorted in non-descending alphabetical order . We are allowed to sort any substring of s , but we cannot sort the entire string at once. The goal is to find the smallest number of operations to achieve a fully...

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LeetCode 3860 - Unique Email Groups

LeetCode Problem 3860 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution LeetCode 3860 - Unique Email Groups Problem Understanding This problem asks us to determine how many distinct email groups exist after applying a specific normalization process to every email address. Each email consists of two parts separated by the '@' character: The local name appears before '@' . The domain name appears after '@' . Before comparing emails,...

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LeetCode 3856 - Trim Trailing Vowels

LeetCode Problem 3856 Difficulty: 🟢 Easy Topics: String Solution LeetCode 3856 - Trim Trailing Vowels Problem Understanding This problem asks us to remove all vowels that appear at the end of a given string. The input is a string s consisting only of lowercase English letters. A vowel is defined as one of the five characters: 'a' , 'e' , 'i' , 'o' , or 'u' . The goal is...

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LeetCode 3853 - Merge Close Characters

LeetCode Problem 3853 Difficulty: 🟡 Medium Topics: Hash Table, String Solution Problem Understanding The problem asks us to repeatedly merge close, equal characters in a string s . A character pair is considered close if the distance between their indices is at most k . When a merge occurs, the right character is removed and the string is updated immediately. Merges are performed one at a time, always choosing the...

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LeetCode 3758 - Convert Number Words to Digits

LeetCode Problem 3758 Difficulty: 🟡 Medium Topics: String, Trie Solution Problem Understanding The problem asks us to parse a string s that contains lowercase English letters and may include concatenated English words representing digits from 0 to 9. Our goal is to extract all valid number words in order and convert them to their corresponding digit, producing a string of digits. The string must be processed from left to right...

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LeetCode 3846 - Total Distance to Type a String Using One Finger

LeetCode Problem 3846 Difficulty: 🟡 Medium Topics: Hash Table, String Solution LeetCode 3846 - Total Distance to Type a String Using One Finger Problem Understanding This problem asks us to calculate the total finger movement required to type a given lowercase string on a special keyboard layout using only one finger. The keyboard is arranged as: q w e r t y u i o p a s d f...

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LeetCode 3844 - Longest Almost-Palindromic Substring

LeetCode Problem 3844 Difficulty: 🟡 Medium Topics: Two Pointers, String, Dynamic Programming Solution LeetCode 3844 - Longest Almost-Palindromic Substring Problem Understanding We are given a string s consisting of lowercase English letters. We must find the length of the longest substring that is almost-palindromic . A substring is almost-palindromic if we can remove exactly one character from that substring and the remaining characters form a palindrome. The key detail is...

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LeetCode 3756 - Concatenate Non-Zero Digits and Multiply by Sum II

LeetCode Problem 3756 Difficulty: 🟡 Medium Topics: Math, String, Prefix Sum Solution Please provide the official LeetCode problem statement or confirm the exact problem details for LeetCode 3756 - Concatenate Non-Zero Digits and Multiply by Sum II . I want to avoid hallucinating the intended optimal approach because this appears to be a very recent/problem-specific question, and correctness matters for the full guide, code, worked examples, and complexity analysis. The...

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LeetCode 3750 - Minimum Number of Flips to Reverse Binary String

LeetCode Problem 3750 Difficulty: 🟢 Easy Topics: Math, Two Pointers, String, Bit Manipulation Solution Problem Understanding The problem gives us a positive integer n , and asks us to work with its binary representation. First, we convert n into a binary string s without leading zeros. Then, we compute the reverse of that binary string. Our goal is to determine the minimum number of bit flips required so that s...

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LeetCode 3841 - Palindromic Path Queries in a Tree

LeetCode Problem 3841 Difficulty: 🔴 Hard Topics: Array, String, Divide and Conquer, Tree, Segment Tree Solution LeetCode 3841: Palindromic Path Queries in a Tree Problem Understanding We are given a tree with n nodes. Each node stores a lowercase English letter. The tree is undirected and connected, so there is exactly one simple path between any two nodes. The problem supports two kinds of operations: Update Change the character assigned...

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LeetCode 3839 - Number of Prefix Connected Groups

LeetCode Problem 3839 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting Solution LeetCode 3839: Number of Prefix Connected Groups Problem Understanding The problem gives us an array of strings words and an integer k . Two words are considered prefix-connected if they share exactly the same first k characters and they come from different indices in the array. A connected group is defined as a set of words where...

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LeetCode 3838 - Weighted Word Mapping

LeetCode Problem 3838 Difficulty: 🟢 Easy Topics: Array, String, Simulation Solution Problem Understanding The problem provides two inputs: words , an array of lowercase English strings. weights , an array of 26 integers where weights[i] represents the weight assigned to the letter corresponding to index i ( 0 -> 'a' , 1 -> 'b' , ..., 25 -> 'z' ). For every word, we must compute its total weight by...

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LeetCode 3749 - Evaluate Valid Expressions

LeetCode Problem 3749 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Divide and Conquer, Stack Solution Problem Understanding The problem presents a simplified version of evaluating nested mathematical expressions in string format. The input expression is either a single integer literal, which may be negative, or a nested operation of the form op(a,b) where op is one of the four basic arithmetic operations: add , sub , mul , or...

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LeetCode 3746 - Minimum String Length After Balanced Removals

LeetCode Problem 3746 Difficulty: 🟡 Medium Topics: String, Stack, Counting Solution Before I write the full guide, I want to verify one detail because this appears to be a very new LeetCode problem and correctness matters here. The examples strongly suggest a key simplification: the answer may reduce to the absolute difference between the total number of 'a' and 'b' characters, because any balanced substring has net contribution 0 ,...

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LeetCode 3803 - Count Residue Prefixes

LeetCode Problem 3803 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem asks us to analyze prefixes of a string s and count how many of them satisfy a specific condition. A prefix is any substring that starts at the first character and extends to some point in the string. For a prefix to be a residue , the number of distinct characters in that prefix must...

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LeetCode 3823 - Reverse Letters Then Special Characters in a String

LeetCode Problem 3823 Difficulty: 🟢 Easy Topics: Two Pointers, String, Simulation Solution LeetCode 3823 - Reverse Letters Then Special Characters in a String Problem Understanding This problem gives us a string s that contains two types of characters: Lowercase English letters ( 'a' to 'z' ) Special characters from the set "!@#$%^&*()" We must perform two independent reversals, in a specific order. First, we extract all lowercase letters from the...

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LeetCode 3817 - Good Indices in a Digit String

LeetCode Problem 3817 Difficulty: 🟡 Medium Topics: Math, String Solution LeetCode 3817 - Good Indices in a Digit String Problem Understanding We are given a string s consisting only of decimal digits. The string length is n , and indices are zero-based. For every index i , we need to determine whether i is a good index . An index is considered good if there exists a substring ending exactly...

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LeetCode 3816 - Lexicographically Smallest String After Deleting Duplicate Characters

LeetCode Problem 3816 Difficulty: 🔴 Hard Topics: Hash Table, String, Stack, Greedy, Monotonic Stack Solution LeetCode 3816 - Lexicographically Smallest String After Deleting Duplicate Characters Problem Understanding We are given a string s consisting only of lowercase English letters. The allowed operation is very specific: if a character currently appears at least twice in the string, we may delete exactly one occurrence of that character. We may repeat this operation...

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LeetCode 3813 - Vowel-Consonant Score

LeetCode Problem 3813 Difficulty: 🟢 Easy Topics: String, Simulation Solution LeetCode 3813 - Vowel-Consonant Score Problem Understanding The problem asks us to compute a score for a given string s . The string may contain lowercase English letters, spaces, and digits. We need to count two quantities: v , the number of vowels in the string. c , the number of consonants in the string. The vowels are exactly the...

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LeetCode 3720 - Lexicographically Smallest Permutation Greater Than Target

LeetCode Problem 3720 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Counting, Enumeration Solution Before I write the full guide, I want to verify one important detail: LeetCode 3720 appears to be a very recent problem , and the exact intended optimal approach depends on the precise judge behavior and hidden constraints. Please confirm that the problem title and statement are exactly: LeetCode 3720 - Lexicographically Smallest Permutation Greater Than...

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LeetCode 3714 - Longest Balanced Substring II

LeetCode Problem 3714 Difficulty: 🟡 Medium Topics: Hash Table, String, Prefix Sum Solution Problem Understanding The problem gives us a string s that contains only three possible characters: 'a' , 'b' , and 'c' . We must find the longest substring where every distinct character in that substring appears the same number of times . A key detail is that the condition only applies to distinct characters present in the...

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LeetCode 3805 - Count Caesar Cipher Pairs

LeetCode Problem 3805 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, String, Counting Solution LeetCode 3805 - Count Caesar Cipher Pairs Problem Understanding We are given an array words containing n lowercase strings. Every string has the same length m . Two strings are considered similar if we can repeatedly apply the following operation to either string: Shift every character forward by one position in the alphabet. The alphabet wraps...

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LeetCode 3799 - Word Squares II

LeetCode Problem 3799 Difficulty: 🟡 Medium Topics: Array, String, Backtracking, Sorting, Enumeration Solution LeetCode 3799 - Word Squares II Problem Understanding We are given an array of distinct 4-letter words. Our goal is to construct every valid word square consisting of exactly four different words: top forms the top row. bottom forms the bottom row. left forms the left column. right forms the right column. Since each word has length...

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LeetCode 3696 - Maximum Distance Between Unequal Words in Array I

LeetCode Problem 3696 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding The problem gives us an array of strings, words , and asks us to find the maximum distance between two indices i and j where the words at those positions are different . More formally, we want to find two indices such that: words[i] != words[j] i < j the value j - i + 1 is as...

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LeetCode 3692 - Majority Frequency Characters

LeetCode Problem 3692 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to analyze the frequency distribution of characters in a string and identify a particular group of characters based on how often they appear. We are given a string s containing only lowercase English letters. For every distinct character, we first count how many times it appears. Characters with the same count belong...

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LeetCode 3794 - Reverse String Prefix

LeetCode Problem 3794 Difficulty: 🟢 Easy Topics: Two Pointers, String Solution LeetCode 3794 - Reverse String Prefix Problem Understanding This problem gives us a string s and an integer k . Our task is to reverse only the first k characters of the string while leaving the remaining characters unchanged. In other words, we take the substring consisting of the first k characters, reverse its order, and then append the...

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LeetCode 3703 - Remove K-Balanced Substrings

LeetCode Problem 3703 Difficulty: 🟡 Medium Topics: String, Stack, Simulation Solution LeetCode 3703 - Remove K-Balanced Substrings Problem Understanding We are given a string s consisting only of '(' and ')' , along with an integer k . A substring is considered k-balanced if it is exactly: "(" repeated k times + ")" repeated k times For example: k = 1 → "()" k = 2 → "(())" k =...

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LeetCode 3557 - Find Maximum Number of Non Intersecting Substrings

LeetCode Problem 3557 Difficulty: 🟡 Medium Topics: Hash Table, String, Dynamic Programming, Greedy Solution Problem Understanding This problem asks us to identify the maximum number of non-intersecting substrings from a given string word such that each substring is at least four characters long and starts and ends with the same character . In other words, we are scanning the string for patterns where the first and last character match and...

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LeetCode 3556 - Sum of Largest Prime Substrings

LeetCode Problem 3556 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Sorting, Number Theory Solution Problem Understanding The problem requires identifying prime numbers from all possible substrings of a given string of digits and summing the three largest unique primes. In simpler terms, given a string s of length up to 10, we must consider every contiguous sequence of digits, convert it to an integer (ignoring any leading zeros), and...

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LeetCode 3519 - Count Numbers with Non-Decreasing Digits

LeetCode Problem 3519 Difficulty: 🔴 Hard Topics: Math, String, Dynamic Programming Solution LeetCode 3519 - Count Numbers with Non-Decreasing Digits Problem Understanding We are given two very large integers l and r as decimal strings and a base b where 2 ≤ b ≤ 10 . For every integer x in the inclusive range [l, r] , we consider its representation in base b . We want to count how...

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LeetCode 3522 - Calculate Score After Performing Instructions

LeetCode Problem 3522 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Simulation Solution Problem Understanding The problem presents a simulated execution environment defined by two parallel arrays, instructions and values , each of length n . Each index i represents a single instruction. The instructions[i] element can either be "add" or "jump" . If the instruction is "add" , the associated values[i] is added to a running score , and...

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LeetCode 3504 - Longest Palindrome After Substring Concatenation II

LeetCode Problem 3504 Difficulty: 🔴 Hard Topics: Two Pointers, String, Dynamic Programming Solution LeetCode 3504, Longest Palindrome After Substring Concatenation II Problem Understanding We are given two strings, s and t . We may choose any substring from s and any substring from t . Either substring is allowed to be empty. After selecting them, we concatenate them in this exact order: substring_from_s + substring_from_t The resulting string must be...

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LeetCode 3445 - Maximum Difference Between Even and Odd Frequency II

LeetCode Problem 3445 Difficulty: 🔴 Hard Topics: String, Sliding Window, Enumeration, Prefix Sum Solution LeetCode 3445 - Maximum Difference Between Even and Odd Frequency II Problem Understanding We are given a string s consisting only of digits '0' through '4' , and an integer k . For any substring subs whose length is at least k , we may choose two characters: Character a must appear an odd number of...

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LeetCode 3412 - Find Mirror Score of a String

LeetCode Problem 3412 Difficulty: 🟡 Medium Topics: Hash Table, String, Stack, Simulation Solution LeetCode 3412 - Find Mirror Score of a String Problem Understanding We are given a lowercase English string s . Every letter has a unique "mirror" letter obtained by reversing the alphabet. For example: 'a' ↔ 'z' 'b' ↔ 'y' 'c' ↔ 'x' and so on. We process the string from left to right. For each position...

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LeetCode 3406 - Find the Lexicographically Largest String From the Box II

LeetCode Problem 3406 Difficulty: 🔴 Hard Topics: Two Pointers, String Solution LeetCode 3406 - Find the Lexicographically Largest String From the Box II Problem Understanding We are given a string word of length n and an integer numFriends . In every round, Alice splits the entire string into exactly numFriends non-empty pieces. Every possible valid split is considered exactly once. Every piece produced by every split is placed into a...

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LeetCode 3760 - Maximum Substrings With Distinct Start

LeetCode Problem 3760 Difficulty: 🟡 Medium Topics: Hash Table, String Solution LeetCode 3760 - Maximum Substrings With Distinct Start Problem Understanding The problem asks us to split a string into as many non-empty substrings as possible, with one important restriction: the first character of every substring must be different from the first character of every other substring. In other words, if we divide the string into several contiguous pieces, and...

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LeetCode 3744 - Find Kth Character in Expanded String

LeetCode Problem 3744 Difficulty: 🟡 Medium Topics: String Solution LeetCode 3744 - Find Kth Character in Expanded String Problem Understanding The problem gives us a string s containing one or more lowercase words separated by single spaces. From this original string, we conceptually build a new string t using a special expansion rule. For each word, the first character appears once, the second character appears twice, the third character appears...

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LeetCode 3734 - Lexicographically Smallest Palindromic Permutation Greater Than Target

LeetCode Problem 3734 Difficulty: 🔴 Hard Topics: Two Pointers, String, Enumeration Solution Problem Understanding We are given two strings, s and target , of the same length n . The goal is to rearrange the characters of s into a palindrome and then choose, among all possible palindromic permutations, the lexicographically smallest one that is strictly greater than target . A string a is lexicographically greater than a string b...

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LeetCode 3707 - Equal Score Substrings

LeetCode Problem 3707 Difficulty: 🟢 Easy Topics: String, Prefix Sum Solution Problem Understanding The problem asks us to determine if a given string s can be split into two non-empty substrings such that the sum of the alphabetic positions of characters in the left substring is equal to the sum in the right substring. Each character 'a' to 'z' maps to values 1 through 26 respectively. The input is a...

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LeetCode 3706 - Maximum Distance Between Unequal Words in Array II

LeetCode Problem 3706 Difficulty: 🟡 Medium Topics: Array, String Solution LeetCode 3706 - Maximum Distance Between Unequal Words in Array II Problem Understanding We are given an array of strings called words . Our goal is to find two different indices i and j such that the strings at those positions are different, meaning words[i] != words[j] . For every valid pair, the distance is defined as: $$j - i...

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LeetCode 3694 - Distinct Points Reachable After Substring Removal

LeetCode Problem 3694 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window, Prefix Sum Solution LeetCode 3694 - Distinct Points Reachable After Substring Removal Problem Understanding We are given a movement string s consisting of the four directions: U increases the y-coordinate by 1. D decreases the y-coordinate by 1. L decreases the x-coordinate by 1. R increases the x-coordinate by 1. Starting from (0, 0) , the entire string...

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LeetCode 3664 - Two-Letter Card Game

LeetCode Problem 3664 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting, Enumeration Solution Problem Understanding Each card consists of exactly two lowercase letters. A card is only relevant if it contains the given letter x somewhere in its two positions. During the game, we repeatedly select two cards that satisfy two conditions: Both cards contain the letter x . The two strings differ in exactly one position. After selecting...

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LeetCode 3662 - Filter Characters by Frequency

LeetCode Problem 3662 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding This problem asks us to filter a string based on character frequency while preserving the original order of characters. We are given a string s consisting only of lowercase English letters and an integer k . The goal is to construct a new string by keeping only those characters whose total frequency in the entire string...

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LeetCode 3612 - Process String with Special Operations I

LeetCode Problem 3612 Difficulty: 🟡 Medium Topics: String, Simulation Solution Problem Understanding The problem asks us to simulate the construction of a string called result by processing an input string s from left to right. The input string contains lowercase English letters and three special operators: * , # , and % . Each character in the input modifies the evolving result string in a specific way. A lowercase letter...

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LeetCode 3614 - Process String with Special Operations II

LeetCode Problem 3614 Difficulty: 🔴 Hard Topics: String, Simulation Solution Problem Understanding The problem defines a transformation process over a string s that contains lowercase letters and three special operation characters: '*' , '#' , and '%' . We process the string from left to right, maintaining a dynamically changing result string. Each lowercase letter is appended to the result. The '*' operation deletes the last character of the result...

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LeetCode 3606 - Coupon Code Validator

LeetCode Problem 3606 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem is asking us to validate a set of coupons based on three criteria: the coupon code must be a non-empty string containing only alphanumeric characters or underscores, the business line must belong to a set of four predefined categories, and the coupon must be active. We are given three arrays of equal length,...

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LeetCode 3597 - Partition String

LeetCode Problem 3597 Difficulty: 🟡 Medium Topics: Hash Table, String, Trie, Simulation Solution LeetCode 3597 - Partition String Problem Understanding We are given a string s and must partition it into segments according to a very specific process. We start at the current position in the string and begin building a segment character by character. As we extend the segment, we continuously check whether that exact segment has already appeared...

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LeetCode 3582 - Generate Tag for Video Caption

LeetCode Problem 3582 Difficulty: 🟢 Easy Topics: String, Simulation Solution Problem Understanding The problem asks us to transform a given caption string into a valid video tag following a strict sequence of rules. The input is a single string caption consisting only of English letters and spaces. These words must be transformed into a single hashtag-style identifier. The transformation has three ordered steps. First, we must merge all words into...

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LeetCode 3579 - Minimum Steps to Convert String with Operations

LeetCode Problem 3579 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to transform one string, word1 , into another string, word2 , using the minimum number of operations. Both strings have equal length and consist only of lowercase English letters. The allowed operations are applied on contiguous substrings of word1 and include replacing a character, swapping two characters, or reversing the entire substring. Each...

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LeetCode 3571 - Find the Shortest Superstring II

LeetCode Problem 3571 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding This problem asks us to construct the shortest string that contains two given strings, s1 and s2 , as substrings. In other words, we are asked to merge s1 and s2 in such a way that no unnecessary characters are added, but both strings appear contiguously somewhere in the resulting string. The inputs, s1 and s2 , are both...

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LeetCode 3563 - Lexicographically Smallest String After Adjacent Removals

LeetCode Problem 3563 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to find the lexicographically smallest string that can be formed by repeatedly removing adjacent pairs of characters in a string s that are consecutive in the alphabet. Consecutive letters can be in either order (e.g., 'a' followed by 'b' or 'b' followed by 'a' ) and the alphabet is considered circular, meaning 'a'...

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LeetCode 3561 - Resulting String After Adjacent Removals

LeetCode Problem 3561 Difficulty: 🟡 Medium Topics: String, Stack, Simulation Solution Problem Understanding The problem asks us to repeatedly remove pairs of adjacent characters in a string that are consecutive in the alphabet, considering the alphabet as circular. "Consecutive in the alphabet" means two characters where the difference in their positions in the alphabet is 1, either forwards or backwards, and 'a' and 'z' are also considered consecutive. The operation...

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LeetCode 3545 - Minimum Deletions for At Most K Distinct Characters

LeetCode Problem 3545 Difficulty: 🟢 Easy Topics: Hash Table, String, Greedy, Sorting, Counting Solution LeetCode 3545 - Minimum Deletions for At Most K Distinct Characters Problem Understanding The problem gives us a string s containing lowercase English letters and an integer k . We are allowed to delete any characters from the string, including multiple occurrences of the same character. After performing the deletions, the resulting string must contain at...

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LeetCode 3541 - Find Most Frequent Vowel and Consonant

LeetCode Problem 3541 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem requires analyzing a string s composed of lowercase English letters and determining the sum of the maximum frequency of vowels and the maximum frequency of consonants. Vowels are defined as 'a' , 'e' , 'i' , 'o' , and 'u' . All other letters are consonants. The frequency of a character is the number...

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LeetCode 3529 - Count Cells in Overlapping Horizontal and Vertical Substrings

LeetCode Problem 3529 Difficulty: 🟡 Medium Topics: Array, String, Rolling Hash, String Matching, Matrix, Hash Function Solution Problem Understanding We are given a character matrix and a pattern string. The key observation is that the problem defines two unusual traversal orders: A horizontal traversal reads the matrix row by row, from left to right. When a row ends, reading continues at the beginning of the next row. A vertical traversal...

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LeetCode 3527 - Find the Most Common Response

LeetCode Problem 3527 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting Solution LeetCode 3527 - Find the Most Common Response Problem Understanding This problem gives us a two dimensional array of strings called responses . Each responses[i] represents all survey responses collected on the i th day. A response is simply a string such as "good" , "ok" , or "bad" . The important detail is that duplicate responses...

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LeetCode 3518 - Smallest Palindromic Rearrangement II

LeetCode Problem 3518 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Combinatorics, Counting Solution Problem Understanding We are given a string s that is guaranteed to already be a palindrome. We may rearrange its characters, but the final rearranged string must also be a palindrome. Among all distinct palindromic permutations that can be formed from the characters of s , we must return the k -th lexicographically smallest one. If...

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LeetCode 3517 - Smallest Palindromic Rearrangement I

LeetCode Problem 3517 Difficulty: 🟡 Medium Topics: String, Sorting, Counting Sort Solution Problem Understanding The problem asks us to take a palindromic string s and return its lexicographically smallest palindromic permutation . In other words, we need to rearrange the characters of s into a palindrome that is the smallest possible if you were to compare it alphabetically with all other palindromic arrangements. The input string s is guaranteed to...

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LeetCode 3503 - Longest Palindrome After Substring Concatenation I

LeetCode Problem 3503 Difficulty: 🟡 Medium Topics: Two Pointers, String, Dynamic Programming, Enumeration Solution LeetCode 3503 - Longest Palindrome After Substring Concatenation I Problem Understanding We are given two strings, s and t , and we are allowed to choose: Any substring of s , including the empty string. Any substring of t , including the empty string. The chosen substring from s must come first, and the chosen substring...

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LeetCode 3499 - Maximize Active Section with Trade I

LeetCode Problem 3499 Difficulty: 🟡 Medium Topics: String, Enumeration Solution Problem Understanding The problem gives a binary string s representing sections of a system, where '1' indicates an active section and '0' indicates an inactive section. We are allowed at most one trade to maximize the number of active sections. A trade consists of two operations applied sequentially on the string: Convert a contiguous block of '1' s surrounded by...

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LeetCode 3501 - Maximize Active Section with Trade II

LeetCode Problem 3501 Difficulty: 🔴 Hard Topics: Array, String, Binary Search, Segment Tree Solution Problem Understanding The problem presents a binary string s where each character represents an active ( '1' ) or inactive ( '0' ) section. For each query, which specifies a substring of s , we are asked to determine the maximum number of active sections after performing at most one trade operation. A trade consists of...

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LeetCode 3498 - Reverse Degree of a String

LeetCode Problem 3498 Difficulty: 🟢 Easy Topics: String, Simulation Solution LeetCode 3498 - Reverse Degree of a String Problem Understanding The problem asks us to compute a special value called the reverse degree of a string. Normally, letters are assigned positions in the alphabet as: 'a' = 1 'b' = 2 ... 'z' = 26 However, this problem uses the reversed alphabet order , where: 'a' = 26 'b' =...

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LeetCode 3491 - Phone Number Prefix

LeetCode Problem 3491 Difficulty: 🟢 Easy Topics: Array, String, Trie, Sorting Solution Problem Understanding The problem is asking us to determine whether any phone number in a given list is a prefix of another phone number. In simpler terms, if you have a list of strings representing numbers, you need to check that no string starts with another string from the list. If all numbers are independent prefixes, the output...

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LeetCode 3484 - Design Spreadsheet

LeetCode Problem 3484 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design, Matrix Solution LeetCode 3484 - Design Spreadsheet Problem Understanding This problem asks us to design a very small spreadsheet system that supports three operations on cells and one operation for evaluating simple formulas. The spreadsheet always contains exactly 26 columns, labeled 'A' through 'Z' , and a configurable number of rows. Each cell can store an integer value....

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LeetCode 3481 - Apply Substitutions

LeetCode Problem 3481 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Depth-First Search, Breadth-First Search, Graph Theory, Topological Sort Solution LeetCode 3481 - Apply Substitutions Problem Understanding This problem gives us a collection of replacement rules and a text containing placeholders. Every placeholder has the format %X% , where X is a single uppercase letter corresponding to one of the replacement keys. The important detail is that replacement values may...

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LeetCode 3474 - Lexicographically Smallest Generated String

LeetCode Problem 3474 Difficulty: 🔴 Hard Topics: String, Greedy, String Matching Solution LeetCode 3474 - Lexicographically Smallest Generated String Problem Understanding We are given two strings: str1 of length n , containing only 'T' and 'F' str2 of length m , containing lowercase English letters We must construct a string word of length n + m - 1 . For every position i in str1 , the substring of word...

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LeetCode 3461 - Check If Digits Are Equal in String After Operations I

LeetCode Problem 3461 Difficulty: 🟢 Easy Topics: Math, String, Simulation, Combinatorics, Number Theory Solution Problem Understanding This problem asks us to repeatedly transform a string of digits by summing consecutive pairs modulo 10, until only two digits remain, and then check whether those two digits are equal. In other words, we start with a string s of length at least 3 and, at each step, generate a new string where...

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LeetCode 3472 - Longest Palindromic Subsequence After at Most K Operations

LeetCode Problem 3472 Difficulty: 🟡 Medium Topics: String, Dynamic Programming Solution Here’s a complete, detailed technical solution guide for LeetCode 3472 following your formatting rules. Problem Understanding The problem asks us to find the length of the longest palindromic subsequence in a string s after performing at most k operations . Each operation allows us to increment or decrement a character by one in the alphabet, with wraparound from 'a'...

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LeetCode 3463 - Check If Digits Are Equal in String After Operations II

LeetCode Problem 3463 Difficulty: 🔴 Hard Topics: Math, String, Combinatorics, Number Theory Solution Problem Understanding The problem asks us to repeatedly transform a string of digits until only two digits remain, and then check if those two digits are the same. The transformation involves taking each consecutive pair of digits, summing them, and taking the result modulo 10 to form a new digit. This process is applied repeatedly, replacing the...

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LeetCode 3460 - Longest Common Prefix After at Most One Removal

LeetCode Problem 3460 Difficulty: 🟡 Medium Topics: Two Pointers, String Solution LeetCode 3460 - Longest Common Prefix After at Most One Removal Problem Understanding We are given two lowercase strings, s and t . We may remove at most one character from s . The removal is optional, so we are also allowed to leave s unchanged. After performing zero or one removal, we want to compute the length of...

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LeetCode 3458 - Select K Disjoint Special Substrings

LeetCode Problem 3458 Difficulty: 🟡 Medium Topics: Hash Table, String, Dynamic Programming, Greedy, Sorting Solution Problem Understanding The problem asks us to determine whether we can select k disjoint special substrings from a given string s . A special substring is defined as a substring where every character in it does not appear anywhere else in the string. Additionally, the substring cannot be the entire string itself. The input consists...

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LeetCode 3456 - Find Special Substring of Length K

LeetCode Problem 3456 Difficulty: 🟢 Easy Topics: String Solution LeetCode 3456 - Find Special Substring of Length K Problem Understanding The problem gives us a string s and an integer k . We need to determine whether there exists a substring of length exactly k that satisfies three conditions. First, every character inside the substring must be identical. In other words, the substring must consist of only one distinct character...

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LeetCode 3448 - Count Substrings Divisible By Last Digit

LeetCode Problem 3448 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution LeetCode 3448 - Count Substrings Divisible By Last Digit Problem Understanding We are given a string s consisting only of decimal digits. Every substring of s represents a non-negative integer, and leading zeros are allowed. For each substring, we look at its last digit. If the last digit is non-zero, we check whether the numeric value of the substring...

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LeetCode 3442 - Maximum Difference Between Even and Odd Frequency I

LeetCode Problem 3442 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding This problem requires analyzing the frequency of characters in a given string and computing a specific difference. You are given a string s consisting only of lowercase English letters. For each character in the string, you can count how many times it appears. Some characters will appear an odd number of times , and some will...

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LeetCode 3441 - Minimum Cost Good Caption

LeetCode Problem 3441 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding We are given a lowercase string caption . We may repeatedly change any character one step forward or backward in the alphabet. Because we can apply this operation multiple times, converting a character x into a character y costs exactly: $$|x - y|$$ where the difference is measured by alphabet position. The goal is to transform the...

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LeetCode 3435 - Frequencies of Shortest Supersequences

LeetCode Problem 3435 Difficulty: 🔴 Hard Topics: Array, String, Bit Manipulation, Graph Theory, Topological Sort, Enumeration Solution Problem Understanding The problem asks us to find all shortest common supersequences (SCS) of a given list of two-character strings words . A shortest common supersequence is a string of minimum length that contains each word in words as a subsequence. Importantly, the output should not include SCSs that are permutations of each...

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LeetCode 3407 - Substring Matching Pattern

LeetCode Problem 3407 Difficulty: 🟢 Easy Topics: String, String Matching Solution LeetCode 3407. Substring Matching Pattern Problem Understanding The problem gives us two strings: s , the source string. p , a pattern string containing exactly one '*' . The special character '*' can represent any sequence of characters, including an empty sequence. The goal is to determine whether there exists some substring of s that can match the pattern...

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LeetCode 3403 - Find the Lexicographically Largest String From the Box I

LeetCode Problem 3403 Difficulty: 🟡 Medium Topics: Two Pointers, String, Enumeration Solution LeetCode 3403 - Find the Lexicographically Largest String From the Box I Problem Understanding We are given a string word of length n and an integer numFriends . In each round, the string is split into exactly numFriends non-empty contiguous pieces. Every distinct way to place the numFriends - 1 cuts is considered a different round. After each...

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LeetCode 3399 - Smallest Substring With Identical Characters II

LeetCode Problem 3399 Difficulty: 🔴 Hard Topics: String, Binary Search Solution Problem Understanding We are given a binary string s consisting only of '0' and '1' , along with an integer numOps . In one operation, we may choose any index and flip the bit at that position. A '0' becomes '1' , and a '1' becomes '0' . Our goal is not to minimize the total number of changes...

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LeetCode 3335 - Total Characters in String After Transformations I

LeetCode Problem 3335 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Dynamic Programming, Counting Solution Problem Understanding The problem gives us a string s and an integer t . We repeatedly apply a transformation exactly t times. During one transformation: Every character from 'a' to 'y' becomes the next character in the alphabet. The character 'z' becomes the string "ab" . After all transformations are complete, we must return the...

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LeetCode 3333 - Find the Original Typed String II

LeetCode Problem 3333 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Prefix Sum Solution Problem Understanding The problem asks us to determine the number of possible original strings that Alice could have intended to type, given a string word that represents the final output and an integer k representing the minimum length of the original string. Because Alice may press keys too long, each character in the output may correspond to...

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LeetCode 3260 - Find the Largest Palindrome Divisible by K

LeetCode Problem 3260 Difficulty: 🔴 Hard Topics: Math, String, Dynamic Programming, Greedy, Number Theory Solution LeetCode 3260 - Find the Largest Palindrome Divisible by K Problem Understanding The problem asks us to construct the largest possible palindrome with exactly n digits such that the resulting number is divisible by k . A palindrome is a number that reads the same forward and backward. For example, 595 , 1221 , and...

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LeetCode 3119 - Maximum Number of Potholes That Can Be Fixed

LeetCode Problem 3119 Difficulty: 🟡 Medium Topics: String, Greedy, Sorting Solution LeetCode 3119 - Maximum Number of Potholes That Can Be Fixed Problem Understanding We are given a road represented as a string containing only two characters: 'x' represents a pothole. '.' represents a smooth section of road. We are also given a repair budget. A repair operation can only be applied to a contiguous group of potholes. If we...

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LeetCode 3135 - Equalize Strings by Adding or Removing Characters at Ends

LeetCode Problem 3135 Difficulty: 🟡 Medium Topics: String, Binary Search, Dynamic Programming, Sliding Window, Hash Function Solution Problem Understanding The problem gives us two strings, initial and target . We want to transform initial into target using the minimum number of operations. In a single operation, we may either: Add one character to the beginning of the string Add one character to the end of the string Remove one character...

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LeetCode 2976 - Minimum Cost to Convert String I

LeetCode Problem 2976 Difficulty: 🟡 Medium Topics: Array, String, Graph Theory, Shortest Path Solution Problem Understanding This problem asks us to transform one string into another while minimizing the total conversion cost. We are given two strings, source and target , both with the same length. Each position in source must eventually become the corresponding character in target . We are also given a list of allowed character transformations. A...

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LeetCode 2999 - Count the Number of Powerful Integers

LeetCode Problem 2999 Difficulty: 🔴 Hard Topics: Math, String, Dynamic Programming Solution Problem Understanding The problem asks us to count all integers within a given range [start, finish] that satisfy two conditions. First, the integer must end with a specific string s , meaning s is a suffix of the number. Second, each digit in the integer cannot exceed a given limit . For example, if limit is 4 ,...

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LeetCode 1960 - Maximum Product of the Length of Two Palindromic Substrings

LeetCode Problem 1960 Difficulty: 🔴 Hard Topics: Two Pointers, String, Rolling Hash, Hash Function Solution LeetCode 1960 - Maximum Product of the Length of Two Palindromic Substrings Problem Understanding The problem asks us to find two non-overlapping odd-length palindromic substrings whose length product is as large as possible. We are given a string s of length up to 10^5 . We must choose two palindromic substrings: The first occupies indices...

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LeetCode 1963 - Minimum Number of Swaps to Make the String Balanced

LeetCode Problem 1963 Difficulty: 🟡 Medium Topics: Two Pointers, String, Stack, Greedy Solution Problem Understanding The problem asks us to transform a given string of brackets into a balanced string with the minimum number of swaps. The input string s consists of n characters, where n is even, and there are exactly n / 2 opening brackets '[' and n / 2 closing brackets ']' . A string is balanced...

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LeetCode 1880 - Check if Word Equals Summation of Two Words

LeetCode Problem 1880 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to determine if the sum of the numerical values of two words equals the numerical value of a third word. Each word consists of lowercase letters from 'a' to 'j' . Each letter has a letter value , which is its zero-based position in the alphabet ( 'a' -> 0 , 'b' -> 1 ,...

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LeetCode 1844 - Replace All Digits with Characters

LeetCode Problem 1844 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding In this problem, we are given a string s where characters at even indices are always lowercase English letters, and characters at odd indices are always digits. The goal is to replace every digit with a new character computed from the character immediately before it. The operation is called shift(c, x) , which means: Start from character c Move...

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LeetCode 3455 - Shortest Matching Substring

LeetCode Problem 3455 Difficulty: 🔴 Hard Topics: Two Pointers, String, Binary Search, String Matching Solution Problem Understanding This problem asks us to find the length of the shortest substring in a string s that matches a pattern p , where p contains exactly two '*' wildcards . Each '*' can match zero or more characters. The substring of s must contain all the letters in p in the correct order,...

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LeetCode 2933 - High-Access Employees

LeetCode Problem 2933 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem gives us a list of employee access records. Each record contains two strings: The employee's name A timestamp in 24-hour "HHMM" format We need to determine which employees accessed the system at least three times within a time window that is strictly less than one hour. The important detail is that exactly 60...

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LeetCode 2490 - Circular Sentence

LeetCode Problem 2490 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to determine whether a given sentence is circular . A sentence is defined as a string of words separated by a single space, with no leading or trailing spaces. Words contain only uppercase and lowercase English letters. The key condition for circularity is that each word's last character must match the first character of the...

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LeetCode 2283 - Check if Number Has Equal Digit Count and Digit Value

LeetCode Problem 2283 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding This problem asks us to verify a self-descriptive property of a string of digits. You are given a string num of length n , where each character is a digit between '0' and '9'. The requirement is that for every index i in the string, the digit i must appear exactly num[i] times in the entire...

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LeetCode 2384 - Largest Palindromic Number

LeetCode Problem 2384 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Counting Solution Problem Understanding The problem asks us to form the largest palindromic integer using the digits from a given string num . A palindrome is a number that reads the same forwards and backwards, like 121 or 7449447 . The input string num consists only of digits ( 0 to 9 ) and may include repeated digits. We...

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LeetCode 3306 - Count of Substrings Containing Every Vowel and K Consonants II

LeetCode Problem 3306 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to count how many substrings of a given string satisfy two conditions simultaneously: The substring contains all five vowels, 'a' , 'e' , 'i' , 'o' , and 'u' , at least once. The substring contains exactly k consonants. The input consists of: A lowercase English string word A non-negative integer...

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LeetCode 2942 - Find Words Containing Character

LeetCode Problem 2942 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding The problem asks us to process an array of lowercase strings words and a single lowercase character x , and return all indices of the words that contain this character. In simpler terms, for each string in the array, we must check whether x appears anywhere in that string. If it does, we record the index of that...

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LeetCode 3227 - Vowels Game in a String

LeetCode Problem 3227 Difficulty: 🟡 Medium Topics: Math, String, Brainteaser, Game Theory Solution Problem Understanding The problem describes a two-player game between Alice and Bob played on a string s . Alice always moves first. The rules for each player are based on vowel counts in substrings: Alice can remove any non-empty substring containing an odd number of vowels , while Bob can remove any non-empty substring containing an even...

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LeetCode 2337 - Move Pieces to Obtain a String

LeetCode Problem 2337 Difficulty: 🟡 Medium Topics: Two Pointers, String Solution Problem Understanding The problem gives us two strings, start and target , both having the same length. Each position in the strings contains one of three characters: 'L' , representing a piece that can only move left 'R' , representing a piece that can only move right '_' , representing an empty space A move consists of sliding a...

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LeetCode 1910 - Remove All Occurrences of a Substring

LeetCode Problem 1910 Difficulty: 🟡 Medium Topics: String, Stack, Simulation Solution Problem Understanding The problem asks us to repeatedly remove occurrences of a substring part from a string s . The important detail is that on every operation, we must remove the leftmost occurrence of part . We continue performing removals until part no longer appears anywhere inside s . In simpler terms, we scan the string, remove one matching...

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LeetCode 2609 - Find the Longest Balanced Substring of a Binary String

LeetCode Problem 2609 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem gives us a binary string s , which means the string contains only the characters '0' and '1' . We need to find the length of the longest substring that is considered balanced. A substring is balanced when two conditions are satisfied: All zeroes appear before all ones. The number of zeroes equals the number of ones....

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LeetCode 2531 - Make Number of Distinct Characters Equal

LeetCode Problem 2531 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Exploration The problem is a pure inequality with a geometric constraint. The condition that $\angle BDC=\frac{\pi}{2}$ gives $$BC^2=BD^2+CD^2$$ by the Pythagorean theorem in $\triangle BDC$. The second condition, that the foot $H$ of the perpendicular from $D$ to plane $ABC$ is the orthocenter of $\triangle ABC$, is much more restrictive than it first appears. It suggests introducing coordinates...

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LeetCode 2825 - Make String a Subsequence Using Cyclic Increments

LeetCode Problem 2825 Difficulty: 🟡 Medium Topics: Two Pointers, String Solution Problem Understanding The problem gives us two lowercase strings, str1 and str2 . We are allowed to perform at most one global operation on str1 . During this operation, we may choose any subset of indices in str1 , and increment the character at each chosen index by one alphabetically. The increment is cyclic, meaning 'z' becomes 'a' ....

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LeetCode 2840 - Check if Strings Can be Made Equal With Operations II

LeetCode Problem 2840 Difficulty: 🟡 Medium Topics: Hash Table, String, Sorting Solution Problem Understanding The problem asks us to determine whether two strings s1 and s2 of equal length can be made identical using a specific type of swap operation. The operation allows us to choose any two indices i and j such that i < j and j - i is even , and then swap the characters at...

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LeetCode 2272 - Substring With Largest Variance

LeetCode Problem 2272 Difficulty: 🔴 Hard Topics: Hash Table, String, Dynamic Programming, Enumeration Solution Problem Understanding This problem asks us to find the maximum possible variance among all substrings of a given string. The string contains only lowercase English letters, and the variance of a substring is defined as the largest difference between the counts of any two characters that both appear in that substring. The important detail is that...

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LeetCode 3167 - Better Compression of String

LeetCode Problem 3167 Difficulty: 🟡 Medium Topics: Hash Table, String, Sorting, Counting Solution Problem Understanding The problem gives us a compressed string where every character is immediately followed by its frequency. For example, the string "a3b2" represents the original expanded string "aaabb" . However, the input compression is not guaranteed to be optimal. The same character may appear multiple times in different parts of the compressed string. For example: "a3c9b2c1"...

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LeetCode 2456 - Most Popular Video Creator

LeetCode Problem 2456 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting, Heap (Priority Queue) Solution Problem Understanding The problem gives us three parallel arrays: creators[i] represents the creator of the i th video ids[i] represents the video ID of the i th video views[i] represents the number of views for the i th video Each index corresponds to one video. Multiple videos can belong to the same creator, and...

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LeetCode 2914 - Minimum Number of Changes to Make Binary String Beautiful

LeetCode Problem 2914 Difficulty: 🟡 Medium Topics: String Solution Problem Understanding You are given a binary string s whose length is guaranteed to be even. A binary string contains only the characters '0' and '1' . A string is considered beautiful if it can be divided into one or more contiguous substrings such that: Every substring has even length. Every substring consists entirely of the same character. For example: "1100"...

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LeetCode 3090 - Maximum Length Substring With Two Occurrences

LeetCode Problem 3090 Difficulty: 🟢 Easy Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem is asking for the maximum length of a substring in a given string s such that no character occurs more than twice within that substring. In other words, for any valid substring, each character can appear at most two times. You need to return the length of the longest such substring. The input...

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LeetCode 2157 - Groups of Strings

LeetCode Problem 2157 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Bit Manipulation, Union-Find Solution Problem Understanding The problem asks us to group a list of unique lowercase strings based on a special notion of connectivity. Each string can be represented as a set of letters , and two strings are connected if one can be transformed into the other with exactly one operation : adding a letter, removing a...

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LeetCode 2486 - Append Characters to String to Make Subsequence

LeetCode Problem 2486 Difficulty: 🟡 Medium Topics: Two Pointers, String, Greedy Solution Problem Understanding The problem asks us to determine the minimum number of characters that need to be appended to the end of string s so that string t becomes a subsequence of s . A subsequence is a sequence that appears in the same order as in another string, but not necessarily consecutively. Essentially, we are allowed to...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1897 - Redistribute Characters to Make All Strings Equal

LeetCode Problem 1897 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks whether it is possible to make all strings in the input array words equal by redistributing characters between them. Specifically, in one operation, you can pick any character from one string and move it to another string at any position. The goal is to determine if, after potentially many such operations, every string...

Practice › LeetCode › LeetCode 32xx ›

LeetCode 3280 - Convert Date to Binary

LeetCode Problem 3280 Difficulty: 🟢 Easy Topics: Math, String Solution LeetCode 3280, Convert Date to Binary Problem Understanding The problem gives us a date string in the format yyyy-mm-dd . The year always has four digits, while the month and day always have two digits because of zero padding. We need to convert each of the three components, year, month, and day, into their binary representations. The important detail is...

Practice › LeetCode › LeetCode 19xx ›

LeetCode 1935 - Maximum Number of Words You Can Type

LeetCode Problem 1935 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem is asking us to determine how many words in a given string can be fully typed on a malfunctioning keyboard. The keyboard has some broken letter keys that cannot be used. The input consists of a string text containing words separated by spaces, and a string brokenLetters containing the distinct letters that are broken. The...

Practice › LeetCode › LeetCode 31xx ›

LeetCode 3136 - Valid Word

LeetCode Problem 3136 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to determine whether a given string qualifies as a "valid word" according to four specific rules. First, the word must contain at least 3 characters. Any string shorter than 3 is automatically invalid. Second, every character in the string must be either: A lowercase English letter An uppercase English letter A digit from 0 to...

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LeetCode 3324 - Find the Sequence of Strings Appeared on the Screen

LeetCode Problem 3324 Difficulty: 🟡 Medium Topics: String, Simulation Solution Problem Understanding The problem gives us a target string and a very restricted keyboard with only two operations. The first key always appends the character "a" to the end of the current string. The second key changes only the last character of the current string to the next letter in the alphabet. For example: "a" becomes "b" "b" becomes "c"...

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LeetCode 2977 - Minimum Cost to Convert String II

LeetCode Problem 2977 Difficulty: 🔴 Hard Topics: Array, String, Dynamic Programming, Graph Theory, Trie, Shortest Path Solution That is a very large, comprehensive request for a Hard problem, with full sections, worked examples, Python and Go implementations, detailed prose, test cases, and edge case analysis. A complete high quality guide for LeetCode 2977 will be long and likely exceed a single clean response. I can provide the complete guide in...

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LeetCode 2330 - Valid Palindrome IV

LeetCode Problem 2330 Difficulty: 🟡 Medium Topics: Two Pointers, String Solution Problem Understanding The problem gives us a string s containing only lowercase English letters. We are allowed to perform operations where we change any single character into any other lowercase character. The goal is to determine whether the string can become a palindrome after performing either exactly one operation or exactly two operations. A palindrome is a string that...

Practice › LeetCode › LeetCode 28xx ›

LeetCode 2801 - Count Stepping Numbers in Range

LeetCode Problem 2801 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to count how many integers in the inclusive range [low, high] are stepping numbers. A stepping number is defined as a number where every pair of adjacent digits differs by exactly 1 . For example: 123454321 is a stepping number because every adjacent pair differs by 1 10 is a stepping number because...

Practice › LeetCode › LeetCode 21xx ›

LeetCode 2166 - Design Bitset

LeetCode Problem 2166 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design Solution Problem Understanding This problem asks us to design a custom Bitset data structure that supports several operations efficiently. A bitset is simply a sequence of binary values, where each position stores either 0 or 1 . We are given a fixed size during initialization, and initially every bit is set to 0 . After that, the class...

Practice › LeetCode › LeetCode 30xx ›

LeetCode 3029 - Minimum Time to Revert Word to Initial State I

LeetCode Problem 3029 Difficulty: 🟡 Medium Topics: String, Rolling Hash, String Matching, Hash Function Solution Problem Understanding The problem gives us a string word and an integer k . Every second, we are forced to perform two operations in sequence: Remove the first k characters from the string. Append any k characters to the end of the string. The appended characters are completely under our control. They do not need...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2288 - Apply Discount to Prices

LeetCode Problem 2288 Difficulty: 🟡 Medium Topics: String Solution Problem Understanding The problem asks us to process a string representing a sentence containing words and prices, where prices are words that start with a dollar sign $ followed by digits. The task is to apply a given percentage discount to every price in the sentence and update the sentence with the discounted prices formatted with exactly two decimal places. Non-price...

Practice › LeetCode › LeetCode 24xx ›

LeetCode 2414 - Length of the Longest Alphabetical Continuous Substring

LeetCode Problem 2414 Difficulty: 🟡 Medium Topics: String Solution Problem Understanding The problem asks us to find the length of the longest substring in a given string s where the characters appear in consecutive alphabetical order. An alphabetical continuous substring means that every adjacent pair of characters differs by exactly one in the alphabet. For example: "abc" is valid because: 'b' comes immediately after 'a' 'c' comes immediately after 'b'...

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LeetCode 3014 - Minimum Number of Pushes to Type Word I

LeetCode Problem 3014 Difficulty: 🟢 Easy Topics: Math, String, Greedy Solution LeetCode 3014 - Minimum Number of Pushes to Type Word I Problem Understanding We are given a word consisting of distinct lowercase English letters. We are allowed to completely redesign the mapping of letters onto the telephone keypad keys 2 through 9 . There are 8 available keys ( 2 to 9 ). Each letter must be assigned to...

Practice › LeetCode › LeetCode 19xx ›

LeetCode 1927 - Sum Game

LeetCode Problem 1927 Difficulty: 🟡 Medium Topics: Math, String, Greedy, Game Theory Solution Problem Understanding The problem describes a two player game played on a numeric string of even length. The string contains digits and possibly some '?' characters. Alice and Bob alternate turns replacing one '?' with a digit from '0' to '9' . Alice moves first. At the end of the game, every '?' has been replaced. The...

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LeetCode 2408 - Design SQL

LeetCode Problem 2408 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design Solution Problem Understanding This problem asks us to design a lightweight in memory database system that supports four core operations on multiple tables: Insert a row into a table Remove a row by id Select a single cell value Export all rows in CSV style format Each table has a fixed number of columns. The constructor receives two...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2264 - Largest 3-Same-Digit Number in String

LeetCode Problem 2264 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem requires identifying the largest "good" integer from a given string of digits. A "good" integer is defined as a substring of length 3 where all digits are identical. The input string num represents a large integer but is provided as a string, which allows us to handle leading zeros and lengths up to 1000 without numeric overflow....

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LeetCode 3292 - Minimum Number of Valid Strings to Form Target II

LeetCode Problem 3292 Difficulty: 🔴 Hard Topics: Array, String, Binary Search, Dynamic Programming, Greedy, Segment Tree, Rolling Hash, String Matching, Hash Function Solution LeetCode 3292 - Minimum Number of Valid Strings to Form Target II Problem Understanding We are given a collection of strings words and a target string target . A string is considered valid if it is a prefix of at least one word in words . This...

Practice › LeetCode › LeetCode 27xx ›

LeetCode 2734 - Lexicographically Smallest String After Substring Operation

LeetCode Problem 2734 Difficulty: 🟡 Medium Topics: String, Greedy Solution Problem Understanding The problem gives us a lowercase English string s . We must perform exactly one operation: Choose any non-empty substring. Replace every character in that substring with the previous character in the alphabet. 'b' becomes 'a' 'c' becomes 'b' 'a' wraps around and becomes 'z' After performing exactly one such operation, we must return the lexicographically smallest possible...

Practice › LeetCode › LeetCode 29xx ›

LeetCode 2953 - Count Complete Substrings

LeetCode Problem 2953 Difficulty: 🔴 Hard Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to count how many substrings of a given string are considered "complete". A substring is complete if it satisfies two conditions simultaneously: Every distinct character in the substring appears exactly k times. For every pair of adjacent characters, the absolute difference between their alphabetical positions is at most 2 . The...

Practice › LeetCode › LeetCode 30xx ›

LeetCode 3016 - Minimum Number of Pushes to Type Word II

LeetCode Problem 3016 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Sorting, Counting Solution LeetCode 3016 - Minimum Number of Pushes to Type Word II Problem Understanding This problem asks us to redesign a telephone keypad so that typing a given word requires the fewest total key presses possible. A traditional telephone keypad contains keys 2 through 9 , giving us exactly 8 available keys . Each lowercase English letter...

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LeetCode 3088 - Make String Anti-palindrome

LeetCode Problem 3088 Difficulty: 🔴 Hard Topics: String, Greedy, Sorting, Counting Sort Solution Problem Understanding We are given a string s of even length, and we may swap any two characters any number of times. Since unrestricted swapping allows us to rearrange the string arbitrarily, the real task is to determine whether some permutation of the characters can form an anti-palindrome. A string is an anti-palindrome if every character differs...

Practice › LeetCode › LeetCode 21xx ›

LeetCode 2156 - Find Substring With Given Hash Value

LeetCode Problem 2156 Difficulty: 🔴 Hard Topics: String, Sliding Window, Rolling Hash, Hash Function Solution Problem Understanding This problem asks us to find the earliest substring of length k whose polynomial rolling hash equals a given target value. The hash function is defined as: $$hash(s, p, m) = \left( \sum_{i=0}^{k-1} val(s[i]) \cdot p^i \right) \bmod m$$ where: val('a') = 1 val('b') = 2 ... val('z') = 26 For a substring...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2063 - Vowels of All Substrings

LeetCode Problem 2063 Difficulty: 🟡 Medium Topics: Math, String, Dynamic Programming, Combinatorics Solution Problem Understanding The problem asks us to compute the total number of vowels that appear across every possible substring of a given string. A substring is any contiguous sequence of characters. For a string of length n , there are n * (n + 1) / 2 total substrings. For each substring, we count how many vowels...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2255 - Count Prefixes of a Given String

LeetCode Problem 2255 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding The problem asks us to determine how many strings in a given array words are prefixes of a target string s . A prefix of a string is defined as any substring that starts at the first character and continues for any length up to the length of the string itself. For example, "ab" is a prefix of...

Practice › LeetCode › LeetCode 33xx ›

LeetCode 3331 - Find Subtree Sizes After Changes

LeetCode Problem 3331 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Tree, Depth-First Search Solution Problem Understanding We are given a rooted tree with n nodes numbered from 0 to n - 1 . The tree structure is represented by the parent array, where parent[i] is the parent of node i . Since node 0 is the root, its parent is -1 . Each node also has a character assigned...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1858 - Longest Word With All Prefixes

LeetCode Problem 1858 Difficulty: 🟡 Medium Topics: Array, String, Depth-First Search, Trie Solution Problem Understanding The problem gives us an array of lowercase strings called words . We need to find the longest word such that every prefix of that word also exists in the array. A prefix means the string formed by taking characters from the beginning of the word. For example, for "apple" : Length 1 prefix: "a"...

Practice › LeetCode › LeetCode 23xx ›

LeetCode 2375 - Construct Smallest Number From DI String

LeetCode Problem 2375 Difficulty: 🟡 Medium Topics: String, Backtracking, Stack, Greedy Solution Problem Understanding The problem asks us to construct the lexicographically smallest number from a string pattern consisting of 'I' and 'D' . Each character in the pattern defines a relationship between consecutive digits of the number: 'I' indicates that the next digit should be larger than the current, and 'D' indicates the next digit should be smaller ....

Practice › LeetCode › LeetCode 30xx ›

LeetCode 3045 - Count Prefix and Suffix Pairs II

LeetCode Problem 3045 Difficulty: 🔴 Hard Topics: Array, String, Trie, Rolling Hash, String Matching, Hash Function Solution Problem Understanding The problem asks us to count the number of ordered pairs (i, j) in a list of strings words such that i < j and words[i] is both a prefix and a suffix of words[j] . In other words, for a pair to be valid, the smaller-indexed string must appear at...

Practice › LeetCode › LeetCode 23xx ›

LeetCode 2379 - Minimum Recolors to Get K Consecutive Black Blocks

LeetCode Problem 2379 Difficulty: 🟢 Easy Topics: String, Sliding Window Solution Problem Understanding The problem gives us a string called blocks , where each character represents the color of a block. A character 'B' means the block is black, and a character 'W' means the block is white. We are also given an integer k . Our goal is to make sure there exists at least one group of exactly...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2262 - Total Appeal of A String

LeetCode Problem 2262 Difficulty: 🔴 Hard Topics: Hash Table, String, Dynamic Programming Solution Problem Understanding The problem asks us to calculate the total appeal of all substrings of a given string s . The appeal of a string is defined as the number of distinct characters it contains. In other words, for every possible substring of s , we need to count how many unique letters it has and sum...

Practice › LeetCode › LeetCode 28xx ›

LeetCode 2896 - Apply Operations to Make Two Strings Equal

LeetCode Problem 2896 Difficulty: 🟡 Medium Topics: String, Dynamic Programming Solution LeetCode 2896 - Apply Operations to Make Two Strings Equal Problem Understanding We are given two binary strings, s1 and s2 , of the same length n , along with a positive integer x . Our goal is to transform s1 into s2 using the minimum possible cost. We are allowed to perform two kinds of operations: Choose any...

Practice › LeetCode › LeetCode 25xx ›

LeetCode 2590 - Design a Todo List

LeetCode Problem 2590 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design, Sorting Solution Problem Understanding This problem asks us to design a small task management system, similar to a lightweight todo application. We need to implement a TodoList class that supports adding tasks, marking tasks as completed, retrieving all pending tasks for a user, and filtering pending tasks by tag. Each task belongs to a specific user and contains...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2052 - Minimum Cost to Separate Sentence Into Rows

LeetCode Problem 2052 Difficulty: 🟡 Medium Topics: String, Dynamic Programming Solution Problem Understanding This problem asks us to format a sentence into multiple rows such that each row has length at most k . The sentence consists of words separated by spaces, and words cannot be split across rows. We may only insert line breaks between words. The goal is not simply to minimize the number of rows. Instead, every...

Practice › LeetCode › LeetCode 21xx ›

LeetCode 2168 - Unique Substrings With Equal Digit Frequency

LeetCode Problem 2168 Difficulty: 🟡 Medium Topics: Hash Table, String, Rolling Hash, Counting, Hash Function Solution Problem Understanding The problem gives us a string s consisting only of digits from '0' to '9' . We must count how many distinct substrings satisfy a special condition: Every digit that appears in the substring must appear the same number of times. A substring is a contiguous portion of the string. The substring...

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LeetCode 2227 - Encrypt and Decrypt Strings

LeetCode Problem 2227 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Design, Trie Solution Problem Understanding This problem asks us to implement an Encrypter class that can both encrypt and decrypt strings according to custom character mappings. The keys array provides the characters that can be encrypted, and values provides the corresponding 2-character strings that each key maps to. The encryption of a string is straightforward: each character is replaced...

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LeetCode 2056 - Number of Valid Move Combinations On Chessboard

LeetCode Problem 2056 Difficulty: 🔴 Hard Topics: Array, String, Backtracking, Simulation Solution Problem Understanding The problem asks us to calculate the total number of valid move combinations for a small set of chess pieces (up to four) on an 8 x 8 chessboard. Each piece-rook, bishop, or queen-can move according to standard chess rules, but only along paths defined by the piece's movement. A move combination consists of assigning a...

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LeetCode 3008 - Find Beautiful Indices in the Given Array II

LeetCode Problem 3008 Difficulty: 🔴 Hard Topics: Two Pointers, String, Binary Search, Rolling Hash, String Matching, Hash Function Solution LeetCode 3008 - Find Beautiful Indices in the Given Array II Problem Understanding This problem asks us to find every starting index where the substring a appears inside the string s , subject to an additional proximity condition involving another substring b . More specifically, an index i is considered beautiful...

Practice › LeetCode › LeetCode 29xx ›

LeetCode 2983 - Palindrome Rearrangement Queries

LeetCode Problem 2983 Difficulty: 🔴 Hard Topics: Hash Table, String, Prefix Sum Solution Problem Understanding We are given a string s of even length n . For every query, we are allowed to independently rearrange two specific substrings: One substring lies completely inside the left half of the string. The other substring lies completely inside the right half of the string. The goal is to determine whether, after rearranging only...

Practice › LeetCode › LeetCode 32xx ›

LeetCode 3216 - Lexicographically Smallest String After a Swap

LeetCode Problem 3216 Difficulty: 🟢 Easy Topics: String, Greedy Solution Problem Understanding The problem requires transforming a string of digits into its lexicographically smallest form by performing at most one swap between adjacent digits of the same parity . Here, parity refers to whether a digit is even or odd. Two digits can only be swapped if both are odd or both are even. The input is a string s...

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LeetCode 3223 - Minimum Length of String After Operations

LeetCode Problem 3223 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to repeatedly remove characters from a string based on a specific rule. Specifically, for any character s[i] in the string, we can remove the nearest occurrence of the same character to the left of i and the nearest occurrence to the right of i . This operation can be applied any number...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2243 - Calculate Digit Sum of a String

LeetCode Problem 2243 Difficulty: 🟢 Easy Topics: String, Simulation Solution Problem Understanding This problem asks us to repeatedly transform a numeric string until its length becomes less than or equal to a given integer k . We are given two inputs: s , a string consisting only of digit characters ( '0' through '9' ) k , an integer that determines the size of groups used during each transformation round...

Practice › LeetCode › LeetCode 33xx ›

LeetCode 3320 - Count The Number of Winning Sequences

LeetCode Problem 3320 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding We are given a string s representing Alice’s moves across n rounds of a game. Each character corresponds to one creature: 'F' = Fire Dragon 'W' = Water Serpent 'E' = Earth Golem In every round, Alice and Bob each choose one creature simultaneously. Depending on the matchup, exactly one player may gain a point: Fire beats...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1850 - Minimum Adjacent Swaps to Reach the Kth Smallest Number

LeetCode Problem 1850 Difficulty: 🟡 Medium Topics: Two Pointers, String, Greedy Solution Problem Understanding The problem asks us to compute the minimum number of adjacent swaps needed to transform a given number num into the kth smallest wonderful integer . A wonderful integer is any permutation of num 's digits that is strictly greater than num . Among all such permutations, we only care about the kth smallest one. The...

Practice › LeetCode › LeetCode 31xx ›

LeetCode 3138 - Minimum Length of Anagram Concatenation

LeetCode Problem 3138 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding The problem gives us a string s that was formed by concatenating several strings together, where every piece is an anagram of the same unknown string t . Our task is to determine the minimum possible length of t . An anagram means the characters are the same, only the order may differ. For example: "abc"...

Practice › LeetCode › LeetCode 26xx ›

LeetCode 2696 - Minimum String Length After Removing Substrings

LeetCode Problem 2696 Difficulty: 🟢 Easy Topics: String, Stack, Simulation Solution Problem Understanding The problem asks us to determine the minimum possible length of a string after repeatedly removing the substrings "AB" or "CD" . We are given a string s consisting solely of uppercase English letters. Each operation allows us to remove any occurrence of "AB" or "CD" from the string, and after removal, the remaining parts of the...

Practice › LeetCode › LeetCode 33xx ›

LeetCode 3316 - Find Maximum Removals From Source String

LeetCode Problem 3316 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, String, Dynamic Programming Solution LeetCode 3316 - Find Maximum Removals From Source String Problem Understanding We are given three inputs: source , the original string of length n pattern , a string that is guaranteed to already be a subsequence of source targetIndices , a sorted list of indices in source that are eligible for removal An operation...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2278 - Percentage of Letter in String

LeetCode Problem 2278 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding This problem asks us to calculate the percentage of a given character, letter , in a string s , and return it as an integer rounded down to the nearest whole percent. Essentially, we need to determine how many times the character occurs, divide by the total number of characters, multiply by 100, and then round down. The input...

Practice › LeetCode › LeetCode 25xx ›

LeetCode 2575 - Find the Divisibility Array of a String

LeetCode Problem 2575 Difficulty: 🟡 Medium Topics: Array, Math, String Solution Problem Understanding The problem gives us a numeric string word and an integer m . For every prefix of the string, we must determine whether that prefix represents a number divisible by m . A prefix word[0...i] means the substring starting at index 0 and ending at index i , inclusive. Since the string contains only digits, every prefix...

Practice › LeetCode › LeetCode 21xx ›

LeetCode 2194 - Cells in a Range on an Excel Sheet

LeetCode Problem 2194 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding This problem is asking us to generate all the Excel-style cell references within a rectangular range given in string format. Each cell is identified by a column letter and a row number, for example "A1" or "K2" . The input string is formatted as "<col1><row1>:<col2><row2>" , where <col1> and <col2> are uppercase letters indicating the starting and ending columns,...

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LeetCode 3003 - Maximize the Number of Partitions After Operations

LeetCode Problem 3003 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Bit Manipulation, Bitmask Solution Problem Understanding The problem asks us to take a string s and an integer k and compute the maximum number of partitions we can create from s under a specific set of operations. First, we are allowed to change at most one character in the string to any other lowercase English letter. After that, we repeatedly...

Practice › LeetCode › LeetCode 27xx ›

LeetCode 2716 - Minimize String Length

LeetCode Problem 2716 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem asks us to minimize the length of a given string s by repeatedly performing two types of deletion operations. In the first operation, we can pick a character at some index and delete the closest identical character to its left if one exists. In the second operation, we can pick a character and delete the...

Practice › LeetCode › LeetCode 32xx ›

LeetCode 3295 - Report Spam Message

LeetCode Problem 3295 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution LeetCode 3295 - Report Spam Message Problem Understanding This problem asks us to determine whether a given message should be classified as spam. We are given two arrays of strings: message , which contains the words appearing in the message. bannedWords , which contains words that are considered banned. A message is considered spam if at least two...

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LeetCode 3389 - Minimum Operations to Make Character Frequencies Equal

LeetCode Problem 3389 Difficulty: 🔴 Hard Topics: Hash Table, String, Dynamic Programming, Counting, Enumeration Solution Problem Understanding The problem requires transforming a given string s into a good string , where a good string is defined as one in which every character appears the same number of times. The operations allowed include deleting a character, inserting a character, or incrementing a character to the next letter in the alphabet (with...

Practice › LeetCode › LeetCode 19xx ›

LeetCode 1945 - Sum of Digits of String After Convert

LeetCode Problem 1945 Difficulty: 🟢 Easy Topics: String, Simulation Solution Problem Understanding This problem asks us to take a string of lowercase English letters and convert it into a numeric value, then repeatedly transform that number by summing its digits. The first step is a deterministic encoding: each character 'a' through 'z' is mapped to its 1-indexed alphabet position, so 'a' = 1 , 'b' = 2 , ..., 'z'...

Practice › LeetCode › LeetCode 28xx ›

LeetCode 2810 - Faulty Keyboard

LeetCode Problem 2810 Difficulty: 🟢 Easy Topics: String, Simulation Solution Problem Understanding In this problem, we are given a string s that represents characters typed on a faulty keyboard. Most characters behave normally and are appended to the current text on the screen. However, whenever the character 'i' is typed, the keyboard does not insert the character itself. Instead, it immediately reverses the entire string that has already been built....

Practice › LeetCode › LeetCode 24xx ›

LeetCode 2489 - Number of Substrings With Fixed Ratio

LeetCode Problem 2489 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Prefix Sum Solution Problem Understanding The problem gives us a binary string s , along with two coprime integers num1 and num2 . We need to count how many non-empty substrings contain 0 s and 1 s in the exact ratio num1 : num2 . If a substring contains: zeroCount zeros oneCount ones then it is valid only if:...

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LeetCode 2900 - Longest Unequal Adjacent Groups Subsequence I

LeetCode Problem 2900 Difficulty: 🟢 Easy Topics: Array, String, Dynamic Programming, Greedy Solution LeetCode 2900 - Longest Unequal Adjacent Groups Subsequence I Problem Understanding We are given two arrays of the same length: words[i] is a string. groups[i] is either 0 or 1 . We want to select a subsequence of words . A subsequence preserves the original order of elements, but we may skip any number of elements. The...

Practice › LeetCode › LeetCode 19xx ›

LeetCode 1985 - Find the Kth Largest Integer in the Array

LeetCode Problem 1985 Difficulty: 🟡 Medium Topics: Array, String, Divide and Conquer, Sorting, Heap (Priority Queue), Quickselect Solution Problem Understanding This problem asks us to find the kth largest value among a list of integers that are represented as strings. Each element in nums is a non-negative integer encoded as a string with no leading zeros, and we are required to return the kth largest value according to numeric order,...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2060 - Check if an Original String Exists Given Two Encoded Strings

LeetCode Problem 2060 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem is asking us to determine whether there exists a single original string that could have been encoded into two different given strings, s1 and s2 . Each encoded string may contain letters and digits. The digits in an encoded string represent the length of some substring(s) in the original string, while letters represent literal characters...

Practice › LeetCode › LeetCode 28xx ›

LeetCode 2833 - Furthest Point From Origin

LeetCode Problem 2833 Difficulty: 🟢 Easy Topics: String, Counting Solution Problem Understanding This problem asks us to determine the maximum possible distance from the origin after performing a sequence of moves on a number line. We start at position 0 , and we are given a string moves , where each character represents one movement instruction. Each move can shift the current position either left ( -1 ) or right...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2269 - Find the K-Beauty of a Number

LeetCode Problem 2269 Difficulty: 🟢 Easy Topics: Math, String, Sliding Window Solution Problem Understanding The problem asks us to calculate the k-beauty of a given integer num . To do this, we treat num as a string and examine every possible contiguous substring of length k . For each substring, we interpret it as an integer and check whether it divides the original number num evenly. The final answer is...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2083 - Substrings That Begin and End With the Same Letter

LeetCode Problem 2083 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Counting, Prefix Sum Solution Problem Understanding The problem asks us to count all substrings in a given string s that start and end with the same character. A substring is a contiguous sequence of characters, so the order and adjacency of characters matter. The input s is a non-empty string consisting of lowercase English letters only, and its length...

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LeetCode 1888 - Minimum Number of Flips to Make the Binary String Alternating

LeetCode Problem 1888 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Sliding Window Solution Problem Understanding The problem asks us to transform a binary string s into an alternating string using the minimum number of flip operations (Type-2), while optionally performing any number of rotations (Type-1). An alternating string is one where no two consecutive characters are the same, so valid alternating strings follow the patterns "010101..." or "101010..." . The...

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LeetCode 3211 - Generate Binary Strings Without Adjacent Zeros

LeetCode Problem 3211 Difficulty: 🟡 Medium Topics: String, Backtracking, Bit Manipulation Solution Problem Understanding The problem asks us to generate every binary string of length n such that no substring of length 2 contains two consecutive zeros. A binary string contains only the characters "0" and "1" . A substring of length 2 means every adjacent pair of characters in the string. The condition says that every such pair must...

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LeetCode 2232 - Minimize Result by Adding Parentheses to Expression

LeetCode Problem 2232 Difficulty: 🟡 Medium Topics: String, Enumeration Solution Problem Understanding The problem presents a string expression in the form "<num1>+<num2>" , where both <num1> and <num2> are positive integers represented as strings. The task is to insert exactly one pair of parentheses such that the left parenthesis appears to the left of the plus sign and the right parenthesis appears to the right of the plus sign ,...

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LeetCode 2351 - First Letter to Appear Twice

LeetCode Problem 2351 Difficulty: 🟢 Easy Topics: Hash Table, String, Bit Manipulation, Counting Solution Problem Understanding The problem gives us a string s containing only lowercase English letters. We need to return the first letter whose second occurrence appears earliest in the string. This detail is extremely important. The problem is not asking for the first character that repeats somewhere later. Instead, it asks which character reaches its second appearance...

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LeetCode 2418 - Sort the People

LeetCode Problem 2418 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Sorting Solution LeetCode 2418, Sort the People Problem Understanding This problem gives us two arrays of equal length: names[i] represents the name of a person heights[i] represents the height of that same person The two arrays are aligned by index, meaning the person at index i has both the name names[i] and the height heights[i] . The goal is...

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LeetCode 3170 - Lexicographically Minimum String After Removing Stars

LeetCode Problem 3170 Difficulty: 🟡 Medium Topics: Hash Table, String, Stack, Greedy, Heap (Priority Queue) Solution Problem Understanding The problem asks us to transform a string s that may contain the '*' character into a lexicographically smallest string by repeatedly applying a deletion operation. Specifically, for each '*' in the string, we must remove it along with the smallest non- '*' character to its left . If there are multiple...

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LeetCode 2108 - Find First Palindromic String in the Array

LeetCode Problem 2108 Difficulty: 🟢 Easy Topics: Array, Two Pointers, String Solution Problem Understanding The problem asks us to examine an array of strings, words , and identify the first string that is a palindrome. A palindrome is a string that reads the same forward and backward. In other words, the first character matches the last, the second matches the second-to-last, and so on. If no string in the array...

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LeetCode 2000 - Reverse Prefix of Word

LeetCode Problem 2000 Difficulty: 🟢 Easy Topics: Two Pointers, String, Stack Solution Problem Understanding The problem gives us a string word and a character ch . We must locate the first occurrence of ch inside the string. Once we find it, we reverse the substring starting at index 0 and ending at that occurrence, inclusive. After reversing that prefix, the remainder of the string stays exactly the same. For example,...

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LeetCode 2514 - Count Anagrams

LeetCode Problem 2514 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Combinatorics, Counting Solution Problem Understanding The problem asks us to count how many distinct anagram sentences can be formed from a given string s . The input string contains one or more words separated by single spaces. An anagram sentence must preserve the structure of the original sentence. Specifically, the i-th word in the new sentence must be a...

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LeetCode 2452 - Words Within Two Edits of Dictionary

LeetCode Problem 2452 Difficulty: 🟡 Medium Topics: Array, String, Trie Solution Problem Understanding The problem asks us to identify words in the queries list that can be transformed into a word in the dictionary list with at most two character edits . Each edit consists of changing a single character to another lowercase English letter. The words in both arrays are guaranteed to have the same length, which simplifies comparison...

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LeetCode 2559 - Count Vowel Strings in Ranges

LeetCode Problem 2559 Difficulty: 🟡 Medium Topics: Array, String, Prefix Sum Solution Problem Understanding The problem gives us an array of strings called words and a list of range queries called queries . For each query [li, ri] , we must determine how many strings in words between indices li and ri , inclusive, both start and end with a vowel. The vowels are limited to the lowercase English vowels:...

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LeetCode 3006 - Find Beautiful Indices in the Given Array I

LeetCode Problem 3006 Difficulty: 🟡 Medium Topics: Two Pointers, String, Binary Search, Rolling Hash, String Matching, Hash Function Solution Problem Understanding The problem gives us a string s and two smaller strings, a and b . We need to find every index i where substring a appears in s , but only if there exists at least one occurrence of substring b close enough to it. More formally, an index...

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LeetCode 3110 - Score of a String

LeetCode Problem 3110 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding This problem asks us to compute the score of a string, where the score is defined as the sum of the absolute differences of ASCII values of consecutive characters in the string. In other words, for a string s of length n , the score is calculated as: $$\text{score}(s) = |s[0] - s[1]| + |s[1] - s[2]| + \dots...

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LeetCode 2430 - Maximum Deletions on a String

LeetCode Problem 2430 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Rolling Hash, String Matching, Hash Function Solution Maximum Deletions on a String Problem Understanding The problem gives us a string s consisting only of lowercase English letters. We want to completely delete the string using the maximum possible number of operations. In a single operation, we have two possible actions: Delete the entire remaining string immediately. Delete only the first...

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LeetCode 2573 - Find the String with LCP

LeetCode Problem 2573 Difficulty: 🔴 Hard Topics: Array, String, Dynamic Programming, Greedy, Union-Find, Matrix Solution Problem Understanding The problem gives us an n x n matrix called lcp , where: lcp[i][j] represents the length of the longest common prefix between: the suffix starting at index i the suffix starting at index j If the unknown string is word , then: suffix i is word[i:] suffix j is word[j:] The task...

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LeetCode 2115 - Find All Possible Recipes from Given Supplies

LeetCode Problem 2115 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Graph Theory, Topological Sort Solution Problem Understanding This problem asks us to determine which recipes can be created when we start with a set of available supplies and are allowed to recursively create additional recipes. Each recipe has a list of required ingredients. Some ingredients are basic supplies that we already own, while others may themselves be recipes. Once...

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LeetCode 2182 - Construct String With Repeat Limit

LeetCode Problem 2182 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Heap (Priority Queue), Counting Solution Problem Understanding The problem asks us to construct a string from the characters of a given string s under a specific constraint: no character can appear more than repeatLimit times consecutively. The goal is not just to create any valid string but to produce the lexicographically largest one possible. Lexicographic ordering is similar to...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1804 - Implement Trie II (Prefix Tree)

LeetCode Problem 1804 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Trie Solution Problem Understanding This problem asks us to implement a Trie data structure, also known as a prefix tree , which is designed to store strings efficiently in a way that allows fast lookups and prefix queries. We are required to implement the following operations: insert(word) : Add a word to the trie. countWordsEqualTo(word) : Count how many...

Practice › LeetCode › LeetCode 23xx ›

LeetCode 2399 - Check Distances Between Same Letters

LeetCode Problem 2399 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding This problem asks us to verify whether a string follows a very specific spacing rule between repeated characters. We are given a string s that contains only lowercase English letters. Every character that appears in the string appears exactly twice. We are also given an integer array distance of size 26 , where each index corresponds...

Practice › LeetCode › LeetCode 19xx ›

LeetCode 1961 - Check If String Is a Prefix of Array

LeetCode Problem 1961 Difficulty: 🟢 Easy Topics: Array, Two Pointers, String Solution Problem Understanding The problem is asking us to determine whether a given string s can be formed by concatenating the first k elements of an array words , where k is some positive integer less than or equal to the length of words . In other words, s must match exactly the prefix of the array when all...

Practice › LeetCode › LeetCode 28xx ›

LeetCode 2842 - Count K-Subsequences of a String With Maximum Beauty

LeetCode Problem 2842 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Greedy, Sorting, Combinatorics Solution Problem Understanding The problem requires us to count k-subsequences of a string s that have the maximum beauty , where beauty is defined as the sum of the frequency of each character in the subsequence. A k-subsequence is a subsequence of length k where all characters are unique. The input string can be up to...

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LeetCode 1933 - Check if String Is Decomposable Into Value-Equal Substrings

LeetCode Problem 1933 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to determine if a given string s , consisting solely of digits '0' through '9' , can be split into consecutive value-equal substrings such that exactly one substring has length 2 and all remaining substrings have length 3. A value-equal substring is a sequence of identical characters, like "111" or "22" . The input is...

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LeetCode 2800 - Shortest String That Contains Three Strings

LeetCode Problem 2800 Difficulty: 🟡 Medium Topics: String, Greedy, Enumeration Solution Problem Understanding The problem asks us to construct a string that contains three given strings a , b , and c as substrings while minimizing its length. If multiple strings satisfy the minimum length condition, the lexicographically smallest one must be returned. In other words, the solution must be the shortest possible string that embeds all three input strings,...

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LeetCode 3106 - Lexicographically Smallest String After Operations With Constraint

LeetCode Problem 3106 Difficulty: 🟡 Medium Topics: String, Greedy Solution LeetCode 3106 - Lexicographically Smallest String After Operations With Constraint Problem Understanding This problem asks us to transform a given string s into another string t such that the total transformation cost does not exceed k . Among all possible valid strings, we must return the lexicographically smallest one. The key detail is how the transformation cost is defined. Each...

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LeetCode 3114 - Latest Time You Can Obtain After Replacing Characters

LeetCode Problem 3114 Difficulty: 🟢 Easy Topics: String, Enumeration Solution Problem Understanding The problem gives us a string representing a time in 12-hour format using the pattern "HH:MM" . Some characters may already be fixed digits, while others are replaced with "?" . Our task is to replace every "?" with a digit so that the resulting time is both valid and as large as possible chronologically. The valid hour...

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LeetCode 2019 - The Score of Students Solving Math Expression

LeetCode Problem 2019 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, String, Dynamic Programming, Stack, Memoization Solution LeetCode 2019, The Score of Students Solving Math Expression Problem Understanding The problem gives us a mathematical expression string s containing only: Single digit numbers 0-9 Addition operators + Multiplication operators * The expression is guaranteed to be valid. For example: 3+5*2 7+3*1*2 Students were supposed to evaluate the expression using the correct...

Practice › LeetCode › LeetCode 21xx ›

LeetCode 2124 - Check if All A's Appears Before All B's

LeetCode Problem 2124 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem gives us a string s that contains only two possible characters, 'a' and 'b' . We must determine whether every 'a' appears before every 'b' . Another way to think about the requirement is this: once we encounter a 'b' , we should never see another 'a' later in the string. If an 'a' appears after a...

Practice › LeetCode › LeetCode 33xx ›

LeetCode 3343 - Count Number of Balanced Permutations

LeetCode Problem 3343 Difficulty: 🔴 Hard Topics: Math, String, Dynamic Programming, Combinatorics Solution Problem Understanding We are given a string num consisting only of digits. We may rearrange the digits in any possible way, but we only count distinct permutations. A permutation is considered balanced if the sum of the digits placed at even indices equals the sum of the digits placed at odd indices. The indices are zero based....

Practice › LeetCode › LeetCode 19xx ›

LeetCode 1967 - Number of Strings That Appear as Substrings in Word

LeetCode Problem 1967 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding The problem asks us to determine how many strings in the array patterns appear as substrings within a given string word . A substring is defined as a contiguous sequence of characters, meaning the characters must appear in order and without gaps inside word . We are given two inputs. The first input, patterns , is a list...

Practice › LeetCode › LeetCode 25xx ›

LeetCode 2516 - Take K of Each Character From Left and Right

LeetCode Problem 2516 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to determine the minimum number of characters to take from either end of a string s consisting only of the letters 'a' , 'b' , and 'c' so that we collect at least k of each character . You are allowed to take characters from the leftmost or rightmost position of...

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LeetCode 2193 - Minimum Number of Moves to Make Palindrome

LeetCode Problem 2193 Difficulty: 🔴 Hard Topics: Two Pointers, String, Greedy, Binary Indexed Tree Solution LeetCode 2193 - Minimum Number of Moves to Make Palindrome Problem Understanding This problem asks us to transform a given string into a palindrome using the minimum number of adjacent swaps. A palindrome is a string that reads the same forward and backward. For example, "abba" and "racecar" are palindromes. The only operation allowed is...

Practice › LeetCode › LeetCode 30xx ›

LeetCode 3083 - Existence of a Substring in a String and Its Reverse

LeetCode Problem 3083 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem-Type Check This is a Type B, “Prove that” problem. The statement to prove is: Among all triangles determined by 100 points in general position, at most 70% are acute. The proposed proof attempts to establish a universal upper bound on the number of acute triangles. It does not need to characterize equality cases or extremal configurations. However, the...

Practice › LeetCode › LeetCode 33xx ›

LeetCode 3387 - Maximize Amount After Two Days of Conversions

LeetCode Problem 3387 Difficulty: 🟡 Medium Topics: Array, String, Depth-First Search, Breadth-First Search, Graph Theory Solution LeetCode 3387 - Maximize Amount After Two Days of Conversions Problem Understanding This problem models currency conversions as graph traversal across two separate days. You begin with exactly 1.0 unit of initialCurrency , and you may perform any number of exchanges on day 1 using the first set of conversion rates, followed by any...

Practice › LeetCode › LeetCode 27xx ›

LeetCode 2712 - Minimum Cost to Make All Characters Equal

LeetCode Problem 2712 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Greedy Solution Problem Understanding The problem gives us a binary string s consisting only of characters '0' and '1' . Our goal is to make every character in the string equal, meaning the final string must become either all '0' characters or all '1' characters. We are allowed to perform two types of operations: Choose an index i and flip...

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LeetCode 3137 - Minimum Number of Operations to Make Word K-Periodic

LeetCode Problem 3137 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to transform a given string word of length n into a k-periodic string using the minimum number of operations. A string is k-periodic if it can be formed by repeating a substring s of length k multiple times. Each operation allows us to pick two substrings of length k that start at...

Practice › LeetCode › LeetCode 31xx ›

LeetCode 3168 - Minimum Number of Chairs in a Waiting Room

LeetCode Problem 3168 Difficulty: 🟢 Easy Topics: String, Simulation Solution LeetCode 3168, Minimum Number of Chairs in a Waiting Room Problem Understanding The problem gives us a string s that represents events happening in a waiting room over time. Each character corresponds to one second. If the character is 'E' , one person enters the room and occupies a chair. If the character is 'L' , one person leaves the...

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LeetCode 2186 - Minimum Number of Steps to Make Two Strings Anagram II

LeetCode Problem 2186 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to find the minimum number of steps required to make two strings, s and t , anagrams of each other. An anagram means that both strings must contain the exact same characters with the same frequency, though their order can differ. In this problem, a "step" allows appending any character to either...

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LeetCode 1941 - Check if All Characters Have Equal Number of Occurrences

LeetCode Problem 1941 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to determine whether a given string s is good , meaning that all characters that appear in the string occur the same number of times . In other words, if you count the frequency of each distinct character, those frequencies should be identical for the string to be considered good. The input...

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LeetCode 2138 - Divide a String Into Groups of Size k

LeetCode Problem 2138 Difficulty: 🟢 Easy Topics: String, Simulation Solution LeetCode 2138 - Divide a String Into Groups of Size k Problem Understanding The problem asks us to split a string into consecutive groups, where every group must contain exactly k characters. We process the string from left to right. The first k characters form the first group, the next k characters form the second group, and so on. Every...

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LeetCode 3146 - Permutation Difference between Two Strings

LeetCode Problem 3146 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem asks us to compute the permutation difference between two strings, s and t . Both strings contain unique characters, and t is a permutation of s . The permutation difference is calculated by taking each character in s , finding its index in both s and t , and summing the absolute differences of these...

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LeetCode 2546 - Apply Bitwise Operations to Make Strings Equal

LeetCode Problem 2546 Difficulty: 🟡 Medium Topics: String, Bit Manipulation Solution Problem Understanding The problem provides two binary strings, s and target , of equal length n . You are allowed to perform a specific bitwise operation on s any number of times, which involves picking two distinct indices i and j and updating s[i] to s[i] OR s[j] and s[j] to s[i] XOR s[j] . The goal is to...

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LeetCode 2416 - Sum of Prefix Scores of Strings

LeetCode Problem 2416 Difficulty: 🔴 Hard Topics: Array, String, Trie, Counting Solution Problem Understanding The problem is asking us to compute a special score for each string in a list of strings. Specifically, for a string term , its score is defined as the number of strings in the array that have term as a prefix. The main task is to compute, for each string in the list, the sum...

Practice › LeetCode › LeetCode 27xx ›

LeetCode 2767 - Partition String Into Minimum Beautiful Substrings

LeetCode Problem 2767 Difficulty: 🟡 Medium Topics: Hash Table, String, Dynamic Programming, Backtracking Solution Problem Understanding The problem asks us to partition a given binary string s into the minimum number of substrings such that each substring is beautiful . A substring is beautiful if it represents a power of 5 in decimal and does not have leading zeros. The string s contains only '0' and '1' characters, and its...

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LeetCode 2468 - Split Message Based on Limit

LeetCode Problem 2468 Difficulty: 🔴 Hard Topics: String, Enumeration Solution LeetCode 2468 - Split Message Based on Limit Problem Understanding This problem asks us to divide a message into multiple parts while respecting a strict maximum length constraint for every part. Each part must include a suffix in the format "<a/b>" , where: a is the current part number, starting from 1 b is the total number of parts The...

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LeetCode 2434 - Using a Robot to Print the Lexicographically Smallest String

LeetCode Problem 2434 Difficulty: 🟡 Medium Topics: Hash Table, String, Stack, Greedy Solution Problem Understanding In this problem, we are given a string s and a robot that manages another temporary string t . Initially, t is empty. The robot repeatedly performs one of two allowed operations: First, it may remove the first character from s and append it to the end of t . Second, it may remove the...

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LeetCode 3081 - Replace Question Marks in String to Minimize Its Value

LeetCode Problem 3081 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Sorting, Heap (Priority Queue), Counting Solution Problem Understanding The problem gives us a string s containing lowercase English letters and the character '?' . Every '?' must be replaced with a lowercase letter so that the resulting string has the minimum possible value. The value of a string is determined by counting repeated occurrences of letters. For every position...

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LeetCode 2496 - Maximum Value of a String in an Array

LeetCode Problem 2496 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding This problem asks us to compute the maximum value of strings in an array according to a specific definition of value. Each string can either be entirely numeric or alphanumeric. If a string consists only of digits, its value is the integer it represents. If it contains any letters, its value is the length of the string. We...

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LeetCode 2301 - Match Substring After Replacement

LeetCode Problem 2301 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, String Matching Solution Problem Understanding The problem gives us a string s , a target string sub , and a list of character replacement rules called mappings . Each mapping [old, new] means that a character old inside sub may be replaced with new . The replacement is optional, and every character in sub can be replaced at most...

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LeetCode 2472 - Maximum Number of Non-overlapping Palindrome Substrings

LeetCode Problem 2472 Difficulty: 🔴 Hard Topics: Two Pointers, String, Dynamic Programming, Greedy Solution Problem Understanding The problem asks us to find the maximum number of non-overlapping palindrome substrings in a given string s such that each substring has a length of at least k . In other words, we are looking to segment the string into valid palindromic blocks, where no two selected blocks overlap and each block is...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1857 - Largest Color Value in a Directed Graph

LeetCode Problem 1857 Difficulty: 🔴 Hard Topics: Hash Table, String, Dynamic Programming, Graph Theory, Topological Sort, Memoization, Counting Solution Problem Understanding The problem asks us to find the largest color value along any path in a directed graph of n nodes, where each node has a color represented by a lowercase English letter. The graph may contain cycles, in which case the function should return -1 . The input consists...

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LeetCode 3365 - Rearrange K Substrings to Form Target String

LeetCode Problem 3365 Difficulty: 🟡 Medium Topics: Hash Table, String, Sorting Solution Problem Understanding This problem asks whether it is possible to rearrange k contiguous equal-length substrings of a string s to form another string t , given that s and t are anagrams. In simpler terms, we are splitting s into k pieces of the same size, then permuting these pieces in any order to see if we can...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2284 - Sender With Largest Word Count

LeetCode Problem 2284 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting Solution Problem Understanding The problem asks us to analyze a chat log containing n messages and identify which sender has written the most words overall. Each message is represented as a string in the messages array, and the corresponding sender of that message is at the same index in the senders array. A word is defined as a...

Practice › LeetCode › LeetCode 29xx ›

LeetCode 2949 - Count Beautiful Substrings II

LeetCode Problem 2949 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Number Theory, Prefix Sum Solution Problem Understanding The problem asks us to find the number of non-empty substrings in a given string s that satisfy two conditions simultaneously. First, the substring must have an equal number of vowels and consonants. Second, the product of the number of vowels and consonants must be divisible by a given integer k ....

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LeetCode 2311 - Longest Binary Subsequence Less Than or Equal to K

LeetCode Problem 2311 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Greedy, Memoization Solution Problem Understanding The problem asks us to find the length of the longest subsequence of a given binary string s such that the resulting binary number is less than or equal to a given integer k . A subsequence is any selection of characters from s in their original order, possibly skipping some. Importantly, leading zeros are...

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LeetCode 2515 - Shortest Distance to Target String in a Circular Array

LeetCode Problem 2515 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding The problem asks us to find the shortest distance from a given starting index in a circular array of strings to a target string. The circular nature of the array means that moving past the end wraps around to the beginning, and moving before the first element wraps around to the last element. This is important because the...

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LeetCode 2851 - String Transformation

LeetCode Problem 2851 Difficulty: 🔴 Hard Topics: Math, String, Dynamic Programming, String Matching Solution Problem Understanding The problem asks us to transform a string s into a target string t using a very specific operation: choosing a non-empty suffix of s (not the whole string) and moving it to the front. The operation can be repeated exactly k times, and we are asked to count the number of distinct sequences...

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LeetCode 3337 - Total Characters in String After Transformations II

LeetCode Problem 3337 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Dynamic Programming, Counting Solution Problem Understanding The problem defines a repeated string transformation process over lowercase English letters. Every character does not simply become one new character, instead it expands into multiple characters depending on the value stored in nums . For a character c , let its alphabet index be: 'a' -> 0 'b' -> 1 ... 'z'...

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LeetCode 2381 - Shifting Letters II

LeetCode Problem 2381 Difficulty: 🟡 Medium Topics: Array, String, Prefix Sum Solution LeetCode 2381 - Shifting Letters II Problem Understanding This problem gives us a lowercase English string s and a list of shift operations. Each operation is represented as: [start, end, direction] The operation affects every character from index start to index end , inclusive. If direction == 1 , every character in that range is shifted forward by...

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LeetCode 3248 - Snake in Matrix

LeetCode Problem 3248 Difficulty: 🟢 Easy Topics: Array, String, Simulation Solution Problem Understanding The problem describes a snake moving inside an n x n grid. Every cell in the grid is assigned a numeric position using the formula: $$\text{grid}[i][j] = (i \times n) + j$$ This means the grid is numbered row by row, starting from the top-left corner. For example, when n = 3 , the grid looks like...

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LeetCode 2030 - Smallest K-Length Subsequence With Occurrences of a Letter

LeetCode Problem 2030 Difficulty: 🔴 Hard Topics: String, Stack, Greedy, Monotonic Stack Solution Problem Understanding The problem asks us to construct the lexicographically smallest subsequence of length k from the string s , while ensuring that a specific character, letter , appears at least repetition times in the resulting subsequence. A subsequence preserves the relative order of characters from the original string, but characters may be skipped. For example, from...

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LeetCode 2068 - Check Whether Two Strings are Almost Equivalent

LeetCode Problem 2068 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to determine whether two strings are "almost equivalent" based on the frequency of each lowercase English letter. We are given two strings, word1 and word2 , both of the same length. For every character from 'a' to 'z' , we compare how many times that character appears in each string. If the...

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LeetCode 2950 - Number of Divisible Substrings

LeetCode Problem 2950 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting, Prefix Sum Solution Problem Understanding The problem asks us to analyze a given string word where each lowercase English letter is mapped to a digit according to a classic phone keypad scheme. A substring of the string is considered divisible if the sum of the mapped digits of its characters is divisible by the length of the substring. The...

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LeetCode 1898 - Maximum Number of Removable Characters

LeetCode Problem 1898 Difficulty: 🟡 Medium Topics: Array, Two Pointers, String, Binary Search Solution Problem Understanding The problem gives us two strings, s and p , and a list of indices removable . The string p is guaranteed to be a subsequence of s , meaning all characters in p appear in s in the same order, though not necessarily consecutively. The array removable represents positions in s that we...

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LeetCode 2325 - Decode the Message

LeetCode Problem 2325 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding This problem is asking us to decode a secret message using a substitution cipher defined by a key string. The key string may include spaces and contains every lowercase letter of the English alphabet at least once. The order in which each letter first appears in the key determines the mapping to the regular alphabet. For example,...

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LeetCode 2353 - Design a Food Rating System

LeetCode Problem 2353 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design, Heap (Priority Queue), Ordered Set Solution Problem Understanding The problem is asking us to implement a food rating system that supports dynamic updates to the ratings of individual food items and allows querying for the highest-rated food for a given cuisine. Each food has a unique name, belongs to exactly one cuisine, and has an associated rating. The...

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LeetCode 2606 - Find the Substring With Maximum Cost

LeetCode Problem 2606 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Dynamic Programming Solution Problem Understanding The problem asks us to find the substring of a given string s that maximizes a custom cost function. Each character in the string has a value: if it exists in the string chars , its value is taken from the corresponding index in the array vals ; if it does not exist in...

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LeetCode 3228 - Maximum Number of Operations to Move Ones to the End

LeetCode Problem 3228 Difficulty: 🟡 Medium Topics: String, Greedy, Counting Solution Problem Understanding The problem gives us a binary string s , consisting only of characters '0' and '1' . We are allowed to repeatedly perform a specific operation: Find an index i such that: s[i] == '1' s[i + 1] == '0' Move that '1' to the right until it either: reaches the end of the string, or stops...

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LeetCode 2167 - Minimum Time to Remove All Cars Containing Illegal Goods

LeetCode Problem 2167 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem presents a binary string s representing a sequence of train cars. Each character '0' or '1' denotes whether a train car contains illegal goods ( '1' ) or not ( '0' ). The goal is to remove all cars containing illegal goods using a combination of three possible operations: removing from the left end, removing...

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LeetCode 2645 - Minimum Additions to Make Valid String

LeetCode Problem 2645 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Stack, Greedy Solution Problem Understanding This problem asks us to determine the minimum number of insertions needed to transform a given string word into a valid string . A valid string is defined as one that can be formed by concatenating the sequence "abc" one or more times. In other words, every valid string follows the repeating pattern: abcabcabc... We...

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LeetCode 2147 - Number of Ways to Divide a Long Corridor

LeetCode Problem 2147 Difficulty: 🔴 Hard Topics: Math, String, Dynamic Programming Solution Problem Understanding The problem presents a long corridor represented as a string, where 'S' denotes a seat and 'P' denotes a plant. The goal is to partition this corridor into sections , such that each section contains exactly two seats and any number of plants. There are already two dividers in place: one before the first character and...

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LeetCode 3298 - Count Substrings That Can Be Rearranged to Contain a String II

LeetCode Problem 3298 Difficulty: 🔴 Hard Topics: Hash Table, String, Sliding Window Solution Problem Understanding We are given two strings, word1 and word2 . We need to count how many substrings of word1 are considered valid. A substring is valid if its characters can be rearranged so that word2 becomes a prefix of the rearranged string. This condition is equivalent to saying that the substring must contain at least all...

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LeetCode 2844 - Minimum Operations to Make a Special Number

LeetCode Problem 2844 Difficulty: 🟡 Medium Topics: Math, String, Greedy, Enumeration Solution Problem Understanding The problem asks us to transform a string num representing a non-negative integer into a special number by deleting as few digits as possible. A special number is defined as an integer divisible by 25. In decimal notation, a number is divisible by 25 if its last two digits are one of 00 , 25 ,...

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LeetCode 3125 - Maximum Number That Makes Result of Bitwise AND Zero

LeetCode Problem 3125 Difficulty: 🟡 Medium Topics: String, Greedy, Sorting Solution Problem Understanding The problem asks us to find the largest integer x such that: x <= n The bitwise AND of every integer in the inclusive range [x, n] equals 0 In other words, we want the widest possible suffix ending at n whose cumulative bitwise AND becomes zero, while keeping x as large as possible. For example, if...

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LeetCode 2478 - Number of Beautiful Partitions

LeetCode Problem 2478 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Prefix Sum Solution Problem Understanding The problem asks us to count the number of ways to split a given string s of digits into exactly k non-overlapping substrings, where each substring satisfies specific rules: it must start with a prime digit ( 2 , 3 , 5 , 7 ), end with a non-prime digit ( 1 , 4 ,...

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LeetCode 2904 - Shortest and Lexicographically Smallest Beautiful String

LeetCode Problem 2904 Difficulty: 🟡 Medium Topics: String, Sliding Window Solution Problem Understanding We are given a binary string s , meaning the string contains only the characters '0' and '1' , along with a positive integer k . The goal is to find a substring of s that is considered beautiful , where a beautiful substring contains exactly k occurrences of '1' . However, simply finding any beautiful substring...

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LeetCode 2423 - Remove Letter To Equalize Frequency

LeetCode Problem 2423 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to determine if we can remove exactly one character from a string word such that all remaining characters in the string have the same frequency . The input string consists of lowercase English letters and has a length between 2 and 100. The output is a boolean indicating whether this transformation is...

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LeetCode 2116 - Check if a Parentheses String Can Be Valid

LeetCode Problem 2116 Difficulty: 🟡 Medium Topics: String, Stack, Greedy Solution Problem Understanding This problem asks whether a given parentheses string can be transformed into a valid parentheses sequence under certain constraints. We are given two strings: s , containing only '(' and ')' locked , containing only '0' and '1' Both strings have the same length n . At every index: If locked[i] == '1' , then s[i] is...

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LeetCode 2719 - Count of Integers

LeetCode Problem 2719 Difficulty: 🔴 Hard Topics: Math, String, Dynamic Programming Solution Problem Understanding The problem asks us to count how many integers lie between two given numeric strings num1 and num2 (inclusive) such that the sum of their digits is between min_sum and max_sum . In other words, we are looking for all integers $x$ where num1 <= x <= num2 and the sum of digits of $x$ falls...

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LeetCode 2828 - Check if a String Is an Acronym of Words

LeetCode Problem 2828 Difficulty: 🟢 Easy Topics: Array, String Solution LeetCode 2828 - Check if a String Is an Acronym of Words Problem Understanding This problem asks us to determine whether a given string s is exactly the acronym formed from an array of words. An acronym is created by taking the first character from each word in the words array and concatenating those characters in the same order as...

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LeetCode 3302 - Find the Lexicographically Smallest Valid Sequence

LeetCode Problem 3302 Difficulty: 🟡 Medium Topics: Two Pointers, String, Dynamic Programming, Greedy Solution Problem Understanding The problem asks us to find a sequence of indices from word1 such that the characters at those indices, when concatenated in order, form a string that is almost equal to word2 . "Almost equal" means we are allowed to change at most one character in the resulting string to make it identical to...

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LeetCode 2287 - Rearrange Characters to Make Target String

LeetCode Problem 2287 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem gives us two strings, s and target . We are allowed to take characters from s and rearrange them in any order to form copies of target . Each character in s can only be used once. The goal is to determine the maximum number of complete copies of target that can be constructed...

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LeetCode 1876 - Substrings of Size Three with Distinct Characters

LeetCode Problem 1876 Difficulty: 🟢 Easy Topics: Hash Table, String, Sliding Window, Counting Solution Problem Understanding The problem asks us to count how many substrings of length exactly three contain only distinct characters. A substring is a continuous portion of the string, so for every position in the string, we can examine the next three consecutive characters and determine whether all three are different. For example, in the string "xyzzaz"...

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LeetCode 2707 - Extra Characters in a String

LeetCode Problem 2707 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Dynamic Programming, Trie Solution Problem Understanding The problem asks us to break a string s into non-overlapping substrings such that each substring exists in a given dictionary . Characters in s that cannot be matched with any dictionary word are considered extra characters . The goal is to find the minimum number of extra characters when splitting s optimally....

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LeetCode 2390 - Removing Stars From a String

LeetCode Problem 2390 Difficulty: 🟡 Medium Topics: String, Stack, Simulation Solution Problem Understanding This problem asks us to repeatedly process a string that contains lowercase English letters and star characters ( * ). Every star represents a removal operation. When we encounter a star, we must remove two things: The star itself. The closest non-star character immediately to its left. We continue this process until all stars have been removed....

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LeetCode 3305 - Count of Substrings Containing Every Vowel and K Consonants I

LeetCode Problem 3305 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to count all substrings of a given string word that satisfy two conditions simultaneously: first, the substring must contain all five vowels 'a' , 'e' , 'i' , 'o' , and 'u' at least once; second, the substring must contain exactly k consonants. The input is a string of lowercase English...

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LeetCode 2062 - Count Vowel Substrings of a String

LeetCode Problem 2062 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem requires counting the number of vowel substrings in a given string word . A substring is any contiguous sequence of characters within the string. A vowel substring is defined as a substring that contains only vowels ( 'a' , 'e' , 'i' , 'o' , 'u' ) and must include all five vowels at least...

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LeetCode 3279 - Maximum Total Area Occupied by Pistons

LeetCode Problem 3279 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Simulation, Counting, Prefix Sum Solution Problem Understanding This problem is about simulating the motion of pistons in a car engine and computing the maximum total area under the pistons over time. Each piston moves either up or down by 1 unit every second, reversing direction when it hits the top ( height ) or bottom ( 0 ). The...

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LeetCode 3297 - Count Substrings That Can Be Rearranged to Contain a String I

LeetCode Problem 3297 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution LeetCode 3297 - Count Substrings That Can Be Rearranged to Contain a String I Problem Understanding We are given two lowercase strings, word1 and word2 . A substring of word1 is considered valid if its characters can be rearranged so that word2 becomes a prefix of the rearranged string. To understand what this means, suppose word2 =...

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LeetCode 2042 - Check if Numbers Are Ascending in a Sentence

LeetCode Problem 2042 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to analyze a sentence represented as a string s and determine whether all numbers embedded in the sentence are strictly increasing from left to right. A sentence here is defined as a sequence of tokens separated by single spaces , where each token is either a word of lowercase English letters or a positive integer...

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LeetCode 1948 - Delete Duplicate Folders in System

LeetCode Problem 1948 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Trie, Hash Function Solution Problem Understanding The problem gives a hierarchical file system represented as a list of absolute paths, where each path is an array of folder names from the root to a leaf folder. From these paths, we must reconstruct the implicit tree structure of folders, where each node represents a folder and edges represent parent-child relationships....

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LeetCode 2135 - Count Words Obtained After Adding a Letter

LeetCode Problem 2135 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Bit Manipulation, Sorting Solution Problem Understanding The problem requires us to determine how many strings in targetWords can be formed from strings in startWords through a specific transformation. The transformation involves two operations: appending a single letter not already present in the string, and then rearranging all letters arbitrarily. Essentially, we are checking whether, by adding one new character...

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LeetCode 3120 - Count the Number of Special Characters I

LeetCode Problem 3120 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem asks us to count the number of special characters in a given string word . A character is defined as special if it appears in both lowercase and uppercase forms within the same string. For example, if 'a' appears as both 'a' and 'A' anywhere in word , then 'a' counts as a special character....

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LeetCode 2451 - Odd String Difference

LeetCode Problem 2451 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution LeetCode 2451 - Odd String Difference Problem Understanding The problem gives us an array of strings where every string has the same length. For each string, we construct a difference array by subtracting the alphabet position of consecutive characters. For example, the string "adc" produces: 'd' - 'a' = 3 'c' - 'd' = -1 So its difference...

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LeetCode 2539 - Count the Number of Good Subsequences

LeetCode Problem 2539 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Combinatorics, Counting Solution

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LeetCode 2437 - Number of Valid Clock Times

LeetCode Problem 2437 Difficulty: 🟢 Easy Topics: String, Enumeration Solution Problem Understanding The problem asks us to determine the number of valid times that can be formed from a partially known digital clock string of the format "hh:mm" , where unknown digits are represented by '?' . Each '?' can be replaced with any digit from 0 to 9 , but the resulting hour ( hh ) must be in...

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LeetCode 2512 - Reward Top K Students

LeetCode Problem 2512 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting, Heap (Priority Queue) Solution Problem Understanding The problem asks us to compute a ranking of students based on textual feedback they receive. We are given two lists of words: positive_feedback and negative_feedback . Every time a student receives a report, we score that report by adding 3 points for each word that appears in positive_feedback and subtracting 1...

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LeetCode 3174 - Clear Digits

LeetCode Problem 3174 Difficulty: 🟢 Easy Topics: String, Stack, Simulation Solution Problem Understanding In this problem, we are given a string s that contains lowercase English letters and digits. The task is to repeatedly remove digits according to a very specific rule. Whenever we encounter a digit, we must delete two characters: The digit itself The closest non-digit character to its left This process continues until all digits are removed....

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LeetCode 1946 - Largest Number After Mutating Substring

LeetCode Problem 1946 Difficulty: 🟡 Medium Topics: Array, String, Greedy Solution Problem Understanding This problem asks us to construct the lexicographically largest possible numeric string after optionally applying a single transformation operation on one contiguous substring of the input string num . Each digit in num can be transformed independently using a fixed mapping array change , where change[d] defines the replacement digit for digit d . We are allowed...

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LeetCode 2211 - Count Collisions on a Road

LeetCode Problem 2211 Difficulty: 🟡 Medium Topics: String, Stack, Simulation Solution LeetCode 2211 - Count Collisions on a Road Problem Understanding The problem describes a one dimensional road containing cars positioned from left to right. Every car has one of three possible states: 'L' , meaning the car moves left 'R' , meaning the car moves right 'S' , meaning the car stays stationary All moving cars travel at the...

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LeetCode 2937 - Make Three Strings Equal

LeetCode Problem 2937 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to make three strings s1 , s2 , and s3 identical by repeatedly deleting their rightmost characters. Each deletion is counted as one operation. The key restriction is that we cannot completely delete a string , so at least one character must remain in each string. If it is impossible to make all three strings...

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LeetCode 1915 - Number of Wonderful Substrings

LeetCode Problem 1915 Difficulty: 🟡 Medium Topics: Hash Table, String, Bit Manipulation, Prefix Sum Solution LeetCode 1915, Number of Wonderful Substrings Problem Understanding The problem asks us to count how many non-empty substrings of a given string are considered "wonderful". A substring is wonderful if at most one character appears an odd number of times inside that substring. The string only contains characters from 'a' to 'j' , which means...

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LeetCode 2938 - Separate Black and White Balls

LeetCode Problem 2938 Difficulty: 🟡 Medium Topics: Two Pointers, String, Greedy Solution Problem Understanding The problem asks us to determine the minimum number of adjacent swaps required to rearrange a string of black and white balls such that all white balls ( 0 s) are grouped on the left and all black balls ( 1 s) are grouped on the right. The input is a binary string s of length...

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LeetCode 2506 - Count Pairs Of Similar Strings

LeetCode Problem 2506 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Bit Manipulation, Counting Solution Problem Understanding The problem gives us an array of strings called words . We must count how many pairs of indices (i, j) satisfy two conditions: i < j words[i] and words[j] are similar Two strings are considered similar if they contain exactly the same set of distinct characters, regardless of order or frequency. For...

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LeetCode 3084 - Count Substrings Starting and Ending with Given Character

LeetCode Problem 3084 Difficulty: 🟡 Medium Topics: Math, String, Counting Solution LeetCode 3084 - Count Substrings Starting and Ending with Given Character Problem Understanding The problem gives us a string s and a character c . We must count how many substrings of s both start and end with the character c . A substring is any contiguous portion of the string. For every possible substring, we check two conditions:...

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LeetCode 1903 - Largest Odd Number in String

LeetCode Problem 1903 Difficulty: 🟢 Easy Topics: Math, String, Greedy Solution Problem Understanding The problem asks us to find the largest-valued odd integer substring from a given string num that represents a large integer. Here, a substring is any contiguous sequence of characters in num . We must return the substring itself as a string, or an empty string if no odd number exists in num . For example, if...

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LeetCode 2744 - Find Maximum Number of String Pairs

LeetCode Problem 2744 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Simulation Solution LeetCode 2744 - Find Maximum Number of String Pairs Problem Understanding This problem gives us an array words containing distinct strings. Every string has length exactly 2 , and all strings consist of lowercase English letters. We can form a pair between two strings words[i] and words[j] when: i < j One string is exactly the reverse...

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LeetCode 1957 - Delete Characters to Make Fancy String

LeetCode Problem 1957 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to transform a given string into a "fancy string". A fancy string is defined as a string that does not contain three consecutive identical characters anywhere in the string. We are allowed to delete characters from the original string, and our goal is to delete as few characters as possible. After performing the minimum deletions,...

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LeetCode 1904 - The Number of Full Rounds You Have Played

LeetCode Problem 1904 Difficulty: 🟡 Medium Topics: Math, String Solution Problem Understanding The problem describes an online chess tournament where a new round begins every 15 minutes. A player can only be credited for a full round if they are present for the entire duration of that round. The input consists of two time strings: loginTime , the moment the player logs in logoutTime , the moment the player logs...

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LeetCode 1974 - Minimum Time to Type Word Using Special Typewriter

LeetCode Problem 1974 Difficulty: 🟢 Easy Topics: String, Greedy Solution Problem Understanding The problem describes a circular typewriter containing all lowercase English letters from 'a' to 'z' . A pointer moves around this circular arrangement, and initially the pointer starts at 'a' . To type a character, the pointer must currently point at that character. Every action costs exactly one second. There are only two possible actions: Move the pointer...

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LeetCode 2296 - Design a Text Editor

LeetCode Problem 2296 Difficulty: 🔴 Hard Topics: Linked List, String, Stack, Design, Simulation, Doubly-Linked List Solution Problem Understanding This problem asks us to design a simplified text editor that supports four operations: Inserting text at the current cursor position Deleting characters to the left of the cursor Moving the cursor left Moving the cursor right The cursor behaves exactly like a cursor in a normal text editor. It always sits...

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LeetCode 1864 - Minimum Number of Swaps to Make the Binary String Alternating

LeetCode Problem 1864 Difficulty: 🟡 Medium Topics: String, Greedy Solution LeetCode 1864 - Minimum Number of Swaps to Make the Binary String Alternating Problem Understanding The problem gives us a binary string s that contains only '0' and '1' . We are allowed to swap any two characters in the string, not necessarily adjacent ones. Our goal is to transform the string into an alternating binary string using the minimum...

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LeetCode 3291 - Minimum Number of Valid Strings to Form Target I

LeetCode Problem 3291 Difficulty: 🟡 Medium Topics: Array, String, Binary Search, Dynamic Programming, Greedy, Trie, Segment Tree, Rolling Hash, String Matching, Hash Function Solution Problem Understanding We are given a list of strings called words and another string called target . The task is to construct target by concatenating several smaller strings, where each smaller string must be a prefix of at least one word in words . A prefix...

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LeetCode 2370 - Longest Ideal Subsequence

LeetCode Problem 2370 Difficulty: 🟡 Medium Topics: Hash Table, String, Dynamic Programming Solution Problem Understanding In this problem, we are given a string s containing only lowercase English letters and an integer k . We need to find the length of the longest subsequence such that the difference between every pair of adjacent characters in that subsequence is at most k . A subsequence does not require characters to be...

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LeetCode 3327 - Check if DFS Strings Are Palindromes

LeetCode Problem 3327 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Tree, Depth-First Search, Hash Function Solution LeetCode 3327 - Check if DFS Strings Are Palindromes Problem Understanding We are given a rooted tree with n nodes. Node 0 is the root, and the parent-child relationships are described by the parent array. Each node i also has an associated character s[i] . For any node x , a DFS traversal...

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LeetCode 2343 - Query Kth Smallest Trimmed Number

LeetCode Problem 2343 Difficulty: 🟡 Medium Topics: Array, String, Divide and Conquer, Sorting, Heap (Priority Queue), Radix Sort, Quickselect Solution Problem Understanding This problem provides a list of numbers represented as strings, all of equal length. The task is to answer multiple queries, where each query asks for the index of the k-th smallest number after trimming every number in the list to its last trimi digits. Importantly, if two...

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LeetCode 2085 - Count Common Words With One Occurrence

LeetCode Problem 2085 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Counting Solution LeetCode 2085, Count Common Words With One Occurrence Problem Understanding This problem gives us two arrays of strings, words1 and words2 . Our task is to count how many strings appear exactly once in both arrays. The key detail is that a word only qualifies if: It appears exactly one time in words1 It appears exactly one...

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LeetCode 3085 - Minimum Deletions to Make String K-Special

LeetCode Problem 3085 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Sorting, Counting Solution Problem Understanding The problem asks us to transform a given string word into a k-special string using the minimum number of deletions. A string is considered k-special if, for every pair of characters in the string, the difference in their frequencies does not exceed k . In simpler terms, all characters' counts must be roughly balanced,...

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LeetCode 3019 - Number of Changing Keys

LeetCode Problem 3019 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to determine how many times a user changes keys while typing a string s . A change of key occurs when the current character typed differs in its lowercase form from the previous character typed. Importantly, differences in case (uppercase vs lowercase) do not count as a key change because modifiers like shift or caps...

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LeetCode 2391 - Minimum Amount of Time to Collect Garbage

LeetCode Problem 2391 Difficulty: 🟡 Medium Topics: Array, String, Prefix Sum Solution Problem Understanding This problem models a garbage collection system with three separate garbage trucks: One truck collects metal garbage, represented by 'M' One truck collects paper garbage, represented by 'P' One truck collects glass garbage, represented by 'G' The input array garbage describes the garbage present at each house. For example, if garbage[i] = "GMP" , then house...

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LeetCode 2053 - Kth Distinct String in an Array

LeetCode Problem 2053 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Counting Solution Problem Understanding The problem asks us to find the kth distinct string in an array of strings. A string is considered distinct if it appears exactly once in the entire array. The important detail is that the order matters. We are not sorting the strings or rearranging them in any way. Instead, we must consider distinct strings...

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LeetCode 2268 - Minimum Number of Keypresses

LeetCode Problem 2268 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Sorting, Counting Solution Problem Understanding The problem asks us to determine the minimum number of keypresses required to type a given string s using a keypad with 9 buttons. Each button can map to at most 3 letters , and each letter must be mapped to exactly one button. When typing a character, the number of presses depends on...

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LeetCode 2405 - Optimal Partition of String

LeetCode Problem 2405 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy Solution Problem Understanding The problem asks us to divide a given string s into the minimum number of substrings such that every substring contains only unique characters . In other words, within a single substring, no character may appear more than once. We are given a string of lowercase English letters and must split it into contiguous pieces. Every...

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LeetCode 3163 - String Compression III

LeetCode Problem 3163 Difficulty: 🟡 Medium Topics: String Solution Problem Understanding The problem asks us to implement a specialized string compression algorithm. Given an input string word , we are required to build a compressed version by repeatedly taking prefixes consisting of consecutive repeating characters, limited to a maximum length of 9 , and appending their length followed by the character to the result string. The process continues until the...

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LeetCode 3330 - Find the Original Typed String I

LeetCode Problem 3330 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding This problem asks us to determine the number of possible original strings Alice intended to type based on the final string displayed on her screen. Alice might have accidentally pressed a key too long at most once , resulting in one sequence of consecutive repeated characters appearing longer than intended. The input is a string word representing the final...

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LeetCode 2864 - Maximum Odd Binary Number

LeetCode Problem 2864 Difficulty: 🟢 Easy Topics: Math, String, Greedy Solution LeetCode 2864 - Maximum Odd Binary Number Problem Understanding The problem gives us a binary string s , containing only '0' and '1' characters. We are allowed to rearrange the bits in any order we want, but we must use exactly the same bits that appear in the original string. Our goal is to create the largest possible binary...

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LeetCode 2785 - Sort Vowels in a String

LeetCode Problem 2785 Difficulty: 🟡 Medium Topics: String, Sorting Solution Problem Understanding This problem requires us to reorder the vowels in a string while keeping all consonants in their original positions. In simpler terms, we are asked to extract all vowels from a string, sort them by their ASCII values, and then put them back in their original positions, leaving consonants untouched. The input s is a string containing only...

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LeetCode 2246 - Longest Path With Different Adjacent Characters

LeetCode Problem 2246 Difficulty: 🔴 Hard Topics: Array, String, Tree, Depth-First Search, Graph Theory, Topological Sort Solution Problem Understanding The problem gives a tree with n nodes labeled 0 to n-1 , rooted at node 0 . The tree is represented using a parent array, where parent[i] indicates the parent of node i . Node 0 has no parent, so parent[0] == -1 . Each node also has a character...

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LeetCode 1832 - Check if the Sentence Is Pangram

LeetCode Problem 1832 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding This problem asks us to determine whether a given string is a pangram. A pangram is a sentence that contains every lowercase English letter, from 'a' through 'z' , at least once. The input is a single string named sentence . The problem guarantees that the string contains only lowercase English letters, which simplifies the implementation because...

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LeetCode 2446 - Determine if Two Events Have Conflict

LeetCode Problem 2446 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding The problem asks us to determine if two events on the same day overlap in time. Each event is given as a pair of strings in HH:MM format representing the start and end times. The output should be a boolean: true if there is any overlap between the two events, and false otherwise. The input guarantees that the...

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LeetCode 2484 - Count Palindromic Subsequences

LeetCode Problem 2484 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to count all palindromic subsequences of length 5 within a given string of digits s . A palindromic subsequence is a sequence that reads the same forward and backward, and a subsequence can be formed by deleting zero or more characters without changing the order of the remaining characters. For example, given "103301"...

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LeetCode 3258 - Count Substrings That Satisfy K-Constraint I

LeetCode Problem 3258 Difficulty: 🟢 Easy Topics: String, Sliding Window Solution LeetCode 3258 - Count Substrings That Satisfy K-Constraint I Problem Understanding The problem gives us a binary string s , meaning the string contains only the characters '0' and '1' , along with an integer k . A substring is considered valid if it satisfies the k-constraint . The constraint is unusual because it uses an OR condition rather...

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LeetCode 3271 - Hash Divided String

LeetCode Problem 3271 Difficulty: 🟡 Medium Topics: String, Simulation Solution Problem Understanding The problem asks us to compute a hashed string from an input string s by dividing it into equal-length substrings and then mapping each substring to a single character using a simple hashing function. Specifically, for each substring of length k , we compute the sum of the positions of its characters in the English alphabet (where 'a'...

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LeetCode 2565 - Subsequence With the Minimum Score

LeetCode Problem 2565 Difficulty: 🔴 Hard Topics: Two Pointers, String, Binary Search Solution This problem is long and deserves a detailed explanation to do it properly, especially with all requested sections, worked examples, Python and Go implementations, proof sketch, comprehensive tests, and edge cases. I can provide the complete LeetCode 2565 guide in one response , but it will be very long. To ensure formatting quality and avoid truncation, I...

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LeetCode 2663 - Lexicographically Smallest Beautiful String

LeetCode Problem 2663 Difficulty: 🔴 Hard Topics: String, Greedy Solution Problem Understanding The problem requires generating the lexicographically smallest beautiful string that is strictly larger than a given beautiful string s . A string is beautiful if it uses only the first k letters of the English alphabet and contains no palindromic substring of length 2 or more. Essentially, no two consecutive letters can be the same, and no three...

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LeetCode 2697 - Lexicographically Smallest Palindrome

LeetCode Problem 2697 Difficulty: 🟢 Easy Topics: Two Pointers, String, Greedy Solution LeetCode 2697, Lexicographically Smallest Palindrome Problem Understanding The problem gives us a string s consisting only of lowercase English letters. We are allowed to replace characters in the string with other lowercase letters. Our goal is to transform the string into a palindrome. A palindrome is a string that reads the same forward and backward. This means that...

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LeetCode 2129 - Capitalize the Title

LeetCode Problem 2129 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to transform the capitalization of every word in a given title string according to a specific rule based on word length. We are given a string called title , which contains one or more words separated by single spaces. Each word contains only English alphabet characters, and there are no leading or trailing spaces. Our...

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LeetCode 2223 - Sum of Scores of Built Strings

LeetCode Problem 2223 Difficulty: 🔴 Hard Topics: String, Binary Search, Rolling Hash, Suffix Array, String Matching, Hash Function Solution Problem Understanding The problem gives us a final string s and describes a process where the string is built by repeatedly prepending characters to the front. This means that every intermediate string si is actually a suffix of the final string s . For example, if: s = "abaca" Then: s1...

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LeetCode 2380 - Time Needed to Rearrange a Binary String

LeetCode Problem 2380 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Simulation Solution Problem Understanding This problem asks us to repeatedly transform a binary string according to a very specific rule. During each second, every occurrence of the substring "01" is replaced simultaneously with "10" . The important detail is that all replacements happen at the same time. This means we do not process one occurrence at a time from left...

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LeetCode 2131 - Longest Palindrome by Concatenating Two Letter Words

LeetCode Problem 2131 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Greedy, Counting Solution Problem Understanding This problem asks us to construct the longest possible palindrome using a list of two-letter words. Each word can be used at most once, and the order of concatenation can be chosen freely. A palindrome reads the same forwards and backwards, which means that for a word pair to appear symmetrically around the center,...

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LeetCode 2483 - Minimum Penalty for a Shop

LeetCode Problem 2483 Difficulty: 🟡 Medium Topics: String, Prefix Sum Solution Problem Understanding The problem asks us to determine the optimal hour to close a shop to minimize a penalty based on customer arrivals. The input is a string customers where each character represents an hour: 'Y' means customers arrive, and 'N' means no customers arrive. The shop can close at any hour from 0 to n (inclusive), and the...

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LeetCode 2586 - Count the Number of Vowel Strings in Range

LeetCode Problem 2586 Difficulty: 🟢 Easy Topics: Array, String, Counting Solution Problem Understanding The problem gives us an array of lowercase strings called words and two integer indices, left and right . We need to count how many strings within the inclusive range [left, right] are considered vowel strings. A string qualifies as a vowel string if both of these conditions are true: The first character is a vowel. The...

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LeetCode 2299 - Strong Password Checker II

LeetCode Problem 2299 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to determine whether a given password string satisfies a set of security requirements. We are given a single string, password , and we must return true if every condition is satisfied, otherwise return false . A password is considered strong only if all of the following are true: The length is at least 8 characters....

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LeetCode 2207 - Maximize Number of Subsequences in a String

LeetCode Problem 2207 Difficulty: 🟡 Medium Topics: String, Greedy, Prefix Sum Solution Problem Understanding This problem asks us to maximize the number of times a two-character string pattern occurs as a subsequence in a given string text , after inserting exactly one character . The character we can insert is restricted to being either pattern[0] or pattern[1] , and it can be inserted at any position, including the beginning or...

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LeetCode 2266 - Count Number of Texts

LeetCode Problem 2266 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Dynamic Programming Solution Problem Understanding The problem requires determining how many possible text messages Alice could have sent given a sequence of digit key presses received by Bob. Each digit from '2' to '9' maps to a set of letters on a phone keypad. Pressing a key multiple times selects a different letter on that key. For example, pressing...

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LeetCode 2802 - Find The K-th Lucky Number

LeetCode Problem 2802 Difficulty: 🟡 Medium Topics: Math, String, Bit Manipulation Solution Problem Understanding This problem asks us to find the k-th lucky number , where a lucky number is defined as an integer consisting only of the digits 4 and 7 . For example, the sequence of lucky numbers in increasing order starts as 4, 7, 44, 47, 74, 77, 444, and so on. The input is a single...

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LeetCode 2075 - Decode the Slanted Ciphertext

LeetCode Problem 2075 Difficulty: 🟡 Medium Topics: String, Simulation Solution Problem Understanding The problem requires decoding a string that was encoded using a slanted transposition cipher . In this cipher, the original text is written diagonally in a grid with a fixed number of rows. Specifically, the placement starts at the top-left and moves down and to the right in a zig-zag manner. After filling the grid, the encoded string...

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LeetCode 3031 - Minimum Time to Revert Word to Initial State II

LeetCode Problem 3031 Difficulty: 🔴 Hard Topics: String, Rolling Hash, String Matching, Hash Function Solution Problem Understanding The problem requires simulating a repetitive transformation on a string word where, at each second, the first k characters are removed, and then any k characters are appended to the end. The key challenge is to determine the minimum number of seconds required for the string to return exactly to its initial state....

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LeetCode 1896 - Minimum Cost to Change the Final Value of Expression

LeetCode Problem 1896 Difficulty: 🔴 Hard Topics: Math, String, Dynamic Programming, Stack Solution Problem Understanding This problem gives us a boolean expression containing only: '0' and '1' binary operators '&' and '|' parentheses The expression is guaranteed to be valid, which means every operator has valid operands and every parenthesis is properly matched. The important detail is that the expression is not evaluated using normal operator precedence. Instead: Parentheses are...

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LeetCode 2450 - Number of Distinct Binary Strings After Applying Operations

LeetCode Problem 2450 Difficulty: 🟡 Medium Topics: Math, String Solution LeetCode 2450 - Number of Distinct Binary Strings After Applying Operations Problem Understanding We are given a binary string s of length n and an integer k . An operation consists of selecting any contiguous substring of length k and flipping every bit in that substring. A 0 becomes 1 , and a 1 becomes 0 . The operation may...

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LeetCode 1881 - Maximum Value after Insertion

LeetCode Problem 1881 Difficulty: 🟡 Medium Topics: String, Greedy Solution Problem Understanding This problem asks us to insert a single digit x into a very large integer n , represented as a string, in a position that maximizes the resulting numerical value. The integer n can be either positive or negative. Since the number may be extremely large, up to 10^5 characters long, we cannot safely convert it to an...

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LeetCode 1977 - Number of Ways to Separate Numbers

LeetCode Problem 1977 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Suffix Array Solution Problem Understanding The problem asks us to determine the number of ways a string of digits, num , can be split into a sequence of positive integers that are non-decreasing and have no leading zeros . Essentially, each valid partition represents a sequence of numbers that could have been originally written down before the commas were removed....

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LeetCode 3213 - Construct String with Minimum Cost

LeetCode Problem 3213 Difficulty: 🔴 Hard Topics: Array, String, Dynamic Programming, Suffix Array Solution Problem Understanding The problem asks us to construct a target string target by repeatedly appending strings from a given array words , where each word has an associated cost in costs . The goal is to determine the minimum total cost to construct target exactly. If it is impossible to form target from the given words,...

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LeetCode 1871 - Jump Game VII

LeetCode Problem 1871 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Sliding Window, Prefix Sum Solution Problem Understanding The problem gives us a binary string s , where each character is either '0' or '1' . We start at index 0 , and the problem guarantees that s[0] == '0' , meaning the starting position is always valid. From any index i , we are allowed to jump to another index...

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LeetCode 3329 - Count Substrings With K-Frequency Characters II

LeetCode Problem 3329 Difficulty: 🔴 Hard Topics: Hash Table, String, Sliding Window Solution LeetCode 3329 - Count Substrings With K-Frequency Characters II Problem Understanding The problem asks us to count how many substrings of a given string s contain at least one character that appears at least k times within that substring. A substring is a contiguous portion of the string. For every possible substring, we need to determine whether...

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LeetCode 2306 - Naming a Company

LeetCode Problem 2306 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Bit Manipulation, Enumeration Solution Problem Understanding The problem gives us a list of unique strings called ideas . We must repeatedly choose two different strings, swap their first characters, and determine whether the newly formed strings are both absent from the original list. If both newly generated names do not already exist in ideas , then the ordered pair...

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LeetCode 3361 - Shift Distance Between Two Strings

LeetCode Problem 3361 Difficulty: 🟡 Medium Topics: Array, String, Prefix Sum Solution Problem Understanding The problem gives us two strings, s and t , of equal length. We want to transform s into t character by character using cyclic alphabet shifts. For every character in s , we are allowed to repeatedly perform one of two operations: Shift the character forward to the next letter in the alphabet. Shift the...

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LeetCode 2911 - Minimum Changes to Make K Semi-palindromes

LeetCode Problem 2911 Difficulty: 🔴 Hard Topics: Two Pointers, String, Dynamic Programming Solution Problem Understanding The problem asks us to partition a given string s into k contiguous substrings and modify the characters minimally so that each substring becomes a semi-palindrome . A semi-palindrome is a string that can be divided into groups based on a repeating pattern length d (a divisor of the string length), where each group forms...

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LeetCode 3143 - Maximum Points Inside the Square

LeetCode Problem 3143 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Binary Search, Sorting Solution Problem Understanding The problem asks us to find the maximum number of points that can be contained in a square centered at the origin (0, 0) such that no two points inside the square share the same tag. The square's edges are parallel to the axes, and points on the edges are considered inside. The...

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LeetCode 3261 - Count Substrings That Satisfy K-Constraint II

LeetCode Problem 3261 Difficulty: 🔴 Hard Topics: Array, String, Binary Search, Sliding Window, Prefix Sum Solution LeetCode 3261 - Count Substrings That Satisfy K-Constraint II Problem Understanding We are given a binary string s , an integer k , and many range queries. For each query [l, r] , we only consider the substring s[l..r] . Among all substrings completely contained inside this range, we must count how many satisfy...

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LeetCode 2185 - Counting Words With a Given Prefix

LeetCode Problem 2185 Difficulty: 🟢 Easy Topics: Array, String, String Matching Solution Problem Understanding The problem gives us two inputs: An array of strings called words A string called pref We must count how many strings inside words start with the string pref . A prefix means the beginning portion of a string. For example: "at" is a prefix of "attention" "pay" is a prefix of "pay" "code" is not...

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LeetCode 2839 - Check if Strings Can be Made Equal With Operations I

LeetCode Problem 2839 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem gives us two strings, s1 and s2 , each containing exactly four lowercase English letters. We are allowed to repeatedly perform a very specific swap operation on either string. In one operation, we may choose two indices i and j such that j - i = 2 , then swap the characters at those positions. Because the...

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LeetCode 3042 - Count Prefix and Suffix Pairs I

LeetCode Problem 3042 Difficulty: 🟢 Easy Topics: Array, String, Trie, Rolling Hash, String Matching, Hash Function Solution LeetCode 3042 - Count Prefix and Suffix Pairs I Problem Understanding The problem gives us an array of strings called words . For every pair of indices (i, j) where i < j , we must determine whether words[i] is both a prefix and a suffix of words[j] . A string is a...

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LeetCode 2901 - Longest Unequal Adjacent Groups Subsequence II

LeetCode Problem 2901 Difficulty: 🟡 Medium Topics: Array, String, Dynamic Programming Solution Problem Understanding The problem asks us to select the longest subsequence of words from a given array such that adjacent words in the subsequence satisfy two conditions: first, their corresponding groups values are different , and second, the words are of the same length and differ by exactly one character (hamming distance = 1). In other words, we...

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LeetCode 3121 - Count the Number of Special Characters II

LeetCode Problem 3121 Difficulty: 🟡 Medium Topics: Hash Table, String Solution Problem Understanding The problem gives us a string word containing uppercase and lowercase English letters. We must count how many letters are considered "special". A character c is special if two conditions are true: The lowercase version of the letter appears somewhere in the string. The uppercase version of the same letter also appears somewhere in the string. Every...

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LeetCode 3274 - Check if Two Chessboard Squares Have the Same Color

LeetCode Problem 3274 Difficulty: 🟢 Easy Topics: Math, String Solution Problem Understanding The problem gives us two chessboard coordinates, such as "a1" or "h8" , and asks whether the two squares have the same color on a standard 8 x 8 chessboard. A chessboard alternates between black and white squares. If one square is black, all horizontally or vertically adjacent squares are white, and vice versa. This alternating pattern means...

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LeetCode 2273 - Find Resultant Array After Removing Anagrams

LeetCode Problem 2273 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Sorting Solution Problem Understanding This problem asks us to repeatedly remove words from an array when two adjacent words are anagrams of each other . We are given a 0-indexed string array words , where every string contains only lowercase English letters. During each operation, we may choose an index i such that the word at words[i] is an...

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LeetCode 3035 - Maximum Palindromes After Operations

LeetCode Problem 3035 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Greedy, Sorting, Counting Solution LeetCode 3035 - Maximum Palindromes After Operations Problem Understanding This problem gives us an array of strings, words , and allows us to swap any character with any other character across the entire collection of strings. The swaps are completely unrestricted. We may swap characters between different strings, within the same string, or perform no...

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LeetCode 2014 - Longest Subsequence Repeated k Times

LeetCode Problem 2014 Difficulty: 🔴 Hard Topics: Hash Table, Two Pointers, String, Backtracking, Counting, Enumeration Solution Problem Understanding The problem asks us to find the longest subsequence of a string s that can be repeated k times while still being a subsequence of s . A subsequence is derived by deleting zero or more characters from a string without changing the order of the remaining characters. A subsequence seq repeated...

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LeetCode 2096 - Step-By-Step Directions From a Binary Tree Node to Another

LeetCode Problem 2096 Difficulty: 🟡 Medium Topics: String, Tree, Depth-First Search, Binary Tree Solution Problem Understanding The problem gives us a binary tree where every node contains a unique value from 1 to n . We are also given two node values: startValue , the node where the path begins destValue , the node where the path must end We must return the shortest sequence of directions needed to move...

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LeetCode 2109 - Adding Spaces to a String

LeetCode Problem 2109 Difficulty: 🟡 Medium Topics: Array, Two Pointers, String, Simulation Solution Problem Understanding The problem requires us to insert spaces into a given string s at specific positions described by the array spaces . Each element in spaces represents an index in the string before which a space should be inserted. For example, if spaces = [2] and s = "hello" , the output would be "he llo"...

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LeetCode 2678 - Number of Senior Citizens

LeetCode Problem 2678 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding This problem asks us to count the number of passengers in a list who are strictly older than 60. Each passenger's information is compressed into a fixed-length string of 15 characters. The string is structured as follows: the first ten characters represent the phone number, the eleventh character represents gender, the twelfth and thirteenth characters represent the age,...

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LeetCode 3348 - Smallest Divisible Digit Product II

LeetCode Problem 3348 Difficulty: 🔴 Hard Topics: Math, String, Backtracking, Greedy, Number Theory Solution Problem Understanding The problem asks us to find the smallest number that satisfies several constraints relative to a given number num and a target integer t . The input num is a string representing a positive integer, and the number must be zero-free , meaning none of its digits are zero. Additionally, the product of its...

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LeetCode 2788 - Split Strings by Separator

LeetCode Problem 2788 Difficulty: 🟢 Easy Topics: Array, String Solution LeetCode 2788 - Split Strings by Separator Problem Understanding This problem asks us to process an array of strings and split each string using a given separator character. After performing all splits, we must collect every resulting substring into a single output array while preserving the original order. More specifically, we are given: words , an array of strings. separator...

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LeetCode 3234 - Count the Number of Substrings With Dominant Ones

LeetCode Problem 3234 Difficulty: 🟡 Medium Topics: String, Enumeration Solution Problem Understanding We are given a binary string s , containing only '0' and '1' . We must count how many substrings satisfy a special condition called "dominant ones". For any substring: Let zeros be the number of '0' characters Let ones be the number of '1' characters The substring is considered valid if: $$\text{ones} \geq (\text{zeros})^2$$ The task is...

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LeetCode 2409 - Count Days Spent Together

LeetCode Problem 2409 Difficulty: 🟢 Easy Topics: Math, String Solution Problem Understanding The problem asks us to calculate the number of days Alice and Bob spend together in Rome based on their respective arrival and departure dates. Each date is represented as a string in the format "MM-DD" . The input consists of four strings: arriveAlice , leaveAlice , arriveBob , and leaveBob . The output should be the total...

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LeetCode 2103 - Rings and Rods

LeetCode Problem 2103 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem is asking us to determine how many rods, labeled from 0 to 9, have all three colors of rings placed on them. Each ring is described by a two-character string: the first character is the color ( 'R' for red, 'G' for green, 'B' for blue) and the second character is the rod index (...

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LeetCode 2114 - Maximum Number of Words Found in Sentences

LeetCode Problem 2114 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding In this problem, we are given an array of strings called sentences . Each string represents a sentence composed of lowercase English words separated by exactly one space. The problem asks us to determine the maximum number of words that appear in any single sentence. A key detail is how words are separated. The problem guarantees that: There...

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LeetCode 1869 - Longer Contiguous Segments of Ones than Zeros

LeetCode Problem 1869 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem gives us a binary string s , which means the string contains only the characters '0' and '1' . We need to determine whether the longest contiguous sequence of 1 s is strictly longer than the longest contiguous sequence of 0 s. A contiguous segment means consecutive characters without interruption. For example, in the string "1110011" :...

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LeetCode 3340 - Check Balanced String

LeetCode Problem 3340 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem gives us a string num that contains only numeric digits from '0' to '9' . We must determine whether the string is "balanced". A string is considered balanced when the sum of digits located at even indices is equal to the sum of digits located at odd indices. Indexing is zero-based, which means: Index 0 is even...

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LeetCode 2086 - Minimum Number of Food Buckets to Feed the Hamsters

LeetCode Problem 2086 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Greedy Solution Problem Understanding The problem is asking us to place the minimum number of food buckets on empty spaces in a linear arrangement of cells so that all hamsters are fed. Each cell in the string hamsters represents either a hamster ( 'H' ) or an empty spot ( '.' ). A hamster can eat if there is at...

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LeetCode 2213 - Longest Substring of One Repeating Character

LeetCode Problem 2213 Difficulty: 🔴 Hard Topics: Array, String, Segment Tree, Ordered Set Solution Problem Understanding The problem gives us an initial string s and a sequence of update operations. Each update changes exactly one character in the string. After every update, we must determine the length of the longest contiguous substring that contains only one repeating character. A substring is contiguous, so characters must appear next to each other....

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LeetCode 2209 - Minimum White Tiles After Covering With Carpets

LeetCode Problem 2209 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Prefix Sum Solution Problem Understanding The problem presents a binary string floor representing a row of tiles, where '0' corresponds to a black tile and '1' corresponds to a white tile. The goal is to cover some of these tiles using a limited number of black carpets of a fixed length, such that the number of white tiles still visible...

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LeetCode 2309 - Greatest English Letter in Upper and Lower Case

LeetCode Problem 2309 Difficulty: 🟢 Easy Topics: Hash Table, String, Enumeration Solution Problem Understanding The problem gives us a string s containing only uppercase and lowercase English letters. Our task is to find the greatest English letter that appears in both lowercase and uppercase forms somewhere in the string. The answer must be returned as an uppercase letter. To understand this clearly, consider what "greatest" means in this context. Letters...

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LeetCode 3076 - Shortest Uncommon Substring in an Array

LeetCode Problem 3076 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie Solution Problem Understanding The problem asks us to find, for each string in a given array, the shortest substring that does not appear in any other string in the array. If multiple shortest substrings exist, we must choose the lexicographically smallest one. If no such substring exists, the answer for that string is an empty string. In other...

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LeetCode 2564 - Substring XOR Queries

LeetCode Problem 2564 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Bit Manipulation Solution Problem Understanding The problem gives us a binary string s and a list of queries. Each query contains two integers, first and second . For every query, we need to find a substring of s whose decimal value satisfies: $$val \oplus first = second$$ where ⊕ represents the bitwise XOR operation. We can rearrange the equation...

Practice › LeetCode › LeetCode 19xx ›

LeetCode 1980 - Find Unique Binary String

LeetCode Problem 1980 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Backtracking Solution Problem Understanding The problem is asking us to find a binary string of length n that does not exist in a given list nums of unique binary strings, where each string in nums also has length n . In simpler terms, we are given n strings that consist of 0s and 1s, and we need to generate...

Practice › LeetCode › LeetCode 30xx ›

LeetCode 3078 - Match Alphanumerical Pattern in Matrix I

LeetCode Problem 3078 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Matrix Solution LeetCode 3078 - Match Alphanumerical Pattern in Matrix I Problem Understanding This problem asks us to search for a rectangular submatrix inside a larger integer matrix, where the submatrix follows a pattern described by digits and lowercase letters. The input consists of: board , a 2D matrix of integers from 0 to 9 pattern , a 2D...

Practice › LeetCode › LeetCode 19xx ›

LeetCode 1930 - Unique Length-3 Palindromic Subsequences

LeetCode Problem 1930 Difficulty: 🟡 Medium Topics: Hash Table, String, Bit Manipulation, Prefix Sum Solution Problem Understanding The problem asks us to count unique palindromic subsequences of length 3 in a given string s . A palindromic string reads the same forwards and backwards, and in this case, we only care about strings of exactly three characters. A subsequence is formed by deleting zero or more characters from the original...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1859 - Sorting the Sentence

LeetCode Problem 1859 Difficulty: 🟢 Easy Topics: String, Sorting Solution Problem Understanding The problem gives us a shuffled sentence , where every word has a number appended to its end. That number represents the word's original position in the sentence, using 1-indexed ordering . For example, the word "This1" means "This" originally appeared in position 1 , while "sentence4" means "sentence" originally appeared in position 4 . The words in...

Practice › LeetCode › LeetCode 21xx ›

LeetCode 2125 - Number of Laser Beams in a Bank

LeetCode Problem 2125 Difficulty: 🟡 Medium Topics: Array, Math, String, Matrix Solution LeetCode 2125 - Number of Laser Beams in a Bank Problem Understanding The problem describes a bank floor plan as a binary matrix, represented by an array of strings. Each row corresponds to one row in the bank, and each character in the string represents a cell. A '1' means there is a security device installed in that...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2259 - Remove Digit From Number to Maximize Result

LeetCode Problem 2259 Difficulty: 🟢 Easy Topics: String, Greedy, Enumeration Solution LeetCode 2259 - Remove Digit From Number to Maximize Result Problem Understanding The problem gives us a string called number , which represents a positive integer, and a character digit , which is guaranteed to appear at least once inside number . Our task is to remove exactly one occurrence of digit from the string so that the resulting...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2055 - Plates Between Candles

LeetCode Problem 2055 Difficulty: 🟡 Medium Topics: Array, String, Binary Search, Prefix Sum Solution Problem Understanding The problem gives us a string s made of two characters: '*' represents a plate '|' represents a candle We are also given multiple queries, where each query specifies a substring of s using indices [left, right] . For every query, we must count how many plates are located between two candles inside that...

Practice › LeetCode › LeetCode 27xx ›

LeetCode 2746 - Decremental String Concatenation

LeetCode Problem 2746 Difficulty: 🟡 Medium Topics: Array, String, Dynamic Programming Solution LeetCode 2746 - Decremental String Concatenation Problem Understanding The problem gives us an array of strings words . We start with words[0] and then process the remaining strings one by one. For each new word words[i] , we have exactly two choices: Append it to the right side of the current string. Append it to the left side...

Practice › LeetCode › LeetCode 19xx ›

LeetCode 1987 - Number of Unique Good Subsequences

LeetCode Problem 1987 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to count how many distinct subsequences of a binary string are considered “good.” A subsequence is any sequence formed by deleting zero or more characters without changing the relative order of remaining characters. However, not every subsequence is valid for this problem. A subsequence is defined as good if it is not empty...

Practice › LeetCode › LeetCode 27xx ›

LeetCode 2710 - Remove Trailing Zeros From a String

LeetCode Problem 2710 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to remove all trailing zeros from a string representing a positive integer. The input num is a string consisting only of digits with no leading zeros, and the length of the string is between 1 and 1000. The output should be a string representation of the same integer but without any zeros at the end....

Practice › LeetCode › LeetCode 29xx ›

LeetCode 2957 - Remove Adjacent Almost-Equal Characters

LeetCode Problem 2957 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Greedy Solution LeetCode 2957 - Remove Adjacent Almost-Equal Characters Problem Understanding We are given a string word consisting of lowercase English letters. Two adjacent characters are considered almost-equal if either: They are exactly the same, such as 'a' and 'a' Their positions in the alphabet differ by exactly one, such as 'a' and 'b' , 'c' and 'b' , 'x'...

Practice › LeetCode › LeetCode 33xx ›

LeetCode 3325 - Count Substrings With K-Frequency Characters I

LeetCode Problem 3325 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to count all substrings of a given string s such that at least one character in the substring appears at least k times . In simpler terms, for every substring we consider, we check whether there is any character that has frequency greater than or equal to k . If there...

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LeetCode 2743 - Count Substrings Without Repeating Character

LeetCode Problem 2743 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to count how many substrings of a given string contain only unique characters. A substring is considered special if no character appears more than once inside that substring. We are given a string s containing only lowercase English letters. For every possible contiguous substring, we need to determine whether all characters...

Practice › LeetCode › LeetCode 23xx ›

LeetCode 2315 - Count Asterisks

LeetCode Problem 2315 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem gives us a string s containing lowercase English letters, vertical bars '|' , and asterisks '*' . The key rule is that every two consecutive vertical bars form a pair. Any asterisk located between the two bars of a pair must be ignored when counting. Our task is to count only the asterisks that are outside these...

Practice › LeetCode › LeetCode 30xx ›

LeetCode 3093 - Longest Common Suffix Queries

LeetCode Problem 3093 Difficulty: 🔴 Hard Topics: Array, String, Trie Solution Problem Understanding The problem asks us to process a set of suffix matching queries efficiently. We are given two arrays: wordsContainer , which contains candidate strings wordsQuery , which contains query strings For every query string, we must find the index of the string in wordsContainer that shares the longest common suffix with that query. A suffix is the...

Practice › LeetCode › LeetCode 33xx ›

LeetCode 3352 - Count K-Reducible Numbers Less Than N

LeetCode Problem 3352 Difficulty: 🔴 Hard Topics: Math, String, Dynamic Programming, Combinatorics Solution Problem Understanding The problem asks us to count the number of positive integers less than n (where n is given in binary as a string s ) that are k-reducible . An integer x is k-reducible if applying the operation "replace x with the count of set bits in its binary representation" at most k times eventually...

Practice › LeetCode › LeetCode 32xx ›

LeetCode 3210 - Find the Encrypted String

LeetCode Problem 3210 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to implement a custom string encryption algorithm. We are given a string s and an integer k . For each character in s , we need to replace it with the character that is k positions ahead in the string, in a cyclic manner. "Cyclic" means that if the index goes beyond the end of...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2038 - Remove Colored Pieces if Both Neighbors are the Same Color

LeetCode Problem 2038 Difficulty: 🟡 Medium Topics: Math, String, Greedy, Game Theory Solution Problem Understanding This problem describes a two-player game played on a line of colored pieces. The input colors is a string representing these pieces, where each character is either 'A' or 'B' . Alice always moves first and can only remove 'A' pieces that are surrounded on both sides by 'A' . Bob moves second and can...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2222 - Number of Ways to Select Buildings

LeetCode Problem 2222 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Prefix Sum Solution Problem Understanding The problem asks us to count the number of ways to select exactly 3 buildings from a street represented as a binary string s , such that no two consecutive buildings among the selected ones have the same type. Here, 0 represents an office, and 1 represents a restaurant. Essentially, for the three selected buildings...

Practice › LeetCode › LeetCode 28xx ›

LeetCode 2868 - The Wording Game

LeetCode Problem 2868 Difficulty: 🔴 Hard Topics: Array, Math, Two Pointers, String, Greedy, Game Theory Solution Problem Understanding The problem describes a turn-based game between Alice and Bob, where both players have a lexicographically sorted list of words. Alice always starts by playing her lexicographically smallest word , and the players then alternate turns. On each turn, the current player must select a word from their list that is closely...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2047 - Number of Valid Words in a Sentence

LeetCode Problem 2047 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding This problem asks us to count the number of valid words in a sentence. The sentence consists of lowercase letters, digits, hyphens, punctuation ( ! , . , , ), and spaces. A word is defined as a token separated by spaces, and it is considered valid if it satisfies three conditions: It contains only lowercase letters, at most...

Practice › LeetCode › LeetCode 29xx ›

LeetCode 2947 - Count Beautiful Substrings I

LeetCode Problem 2947 Difficulty: 🟡 Medium Topics: Hash Table, Math, String, Enumeration, Number Theory, Prefix Sum Solution LeetCode 2947 - Count Beautiful Substrings I Problem Understanding We are given a lowercase English string s and a positive integer k . We must count how many non-empty substrings satisfy the definition of a beautiful substring. For a substring, let: vowels be the number of vowel characters ( a , e ,...

Practice › LeetCode › LeetCode 25xx ›

LeetCode 2522 - Partition String Into Substrings With Values at Most K

LeetCode Problem 2522 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Greedy Solution LeetCode 2522 - Partition String Into Substrings With Values at Most K Problem Understanding The problem gives us a numeric string s , where every character is a digit between '1' and '9' , and an integer k . We must split the string into contiguous substrings such that every substring, when interpreted as an integer, has a...

Practice › LeetCode › LeetCode 27xx ›

LeetCode 2781 - Length of the Longest Valid Substring

LeetCode Problem 2781 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Sliding Window Solution LeetCode 2781 - Length of the Longest Valid Substring Problem Understanding The problem gives us a string word and a list of forbidden strings forbidden . A substring is considered valid if none of its internal substrings appear in the forbidden list. In other words, for a candidate substring word[l:r] , every contiguous segment inside it...

Practice › LeetCode › LeetCode 31xx ›

LeetCode 3144 - Minimum Substring Partition of Equal Character Frequency

LeetCode Problem 3144 Difficulty: 🟡 Medium Topics: Hash Table, String, Dynamic Programming, Counting Solution Problem Understanding In this problem, we are given a lowercase English string s , and we must split it into one or more contiguous substrings such that every substring is balanced . A substring is considered balanced when every distinct character inside it appears the same number of times. The actual frequency does not matter, only...

Practice › LeetCode › LeetCode 29xx ›

LeetCode 2982 - Find Longest Special Substring That Occurs Thrice II

LeetCode Problem 2982 Difficulty: 🟡 Medium Topics: Hash Table, String, Binary Search, Sliding Window, Counting Solution LeetCode 2982 - Find Longest Special Substring That Occurs Thrice II Problem Understanding The problem asks us to find the maximum length of a special substring that appears at least three times in the given string. A substring is considered special if it consists entirely of a single repeated character. Examples include: "a" "bbb"...

Practice › LeetCode › LeetCode 33xx ›

LeetCode 3303 - Find the Occurrence of First Almost Equal Substring

LeetCode Problem 3303 Difficulty: 🔴 Hard Topics: String, String Matching Solution Problem Understanding We are given two strings, s and pattern . The task is to find the smallest starting index in s such that the substring of length len(pattern) is "almost equal" to pattern . Two strings are considered almost equal if we can modify at most one character in one string to make the two strings identical. That...

Practice › LeetCode › LeetCode 27xx ›

LeetCode 2730 - Find the Longest Semi-Repetitive Substring

LeetCode Problem 2730 Difficulty: 🟡 Medium Topics: String, Sliding Window Solution Problem Understanding The problem gives us a string s consisting only of digits from 0 to 9 . We need to find the length of the longest substring that is considered semi-repetitive . A substring is semi-repetitive if it contains at most one pair of equal adjacent digits. An adjacent pair means two neighboring characters that are the same....

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1849 - Splitting a String Into Descending Consecutive Values

LeetCode Problem 1849 Difficulty: 🟡 Medium Topics: String, Backtracking, Enumeration Solution Problem Understanding The problem gives us a numeric string s and asks whether it can be split into at least two non-empty substrings such that their integer values form a strictly descending sequence where every adjacent pair differs by exactly 1 . The important detail is that we compare the numerical values of the substrings, not the literal substring...

Practice › LeetCode › LeetCode 29xx ›

LeetCode 2955 - Number of Same-End Substrings

LeetCode Problem 2955 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting, Prefix Sum Solution Problem Understanding The problem asks us to compute the number of same-end substrings within specified subranges of a given string s . A substring is same-end if its first and last character are identical. We are given multiple queries in the form [li, ri] , each representing a substring s[li..ri] . For each query, we...

Practice › LeetCode › LeetCode 31xx ›

LeetCode 3104 - Find Longest Self-Contained Substring

LeetCode Problem 3104 Difficulty: 🔴 Hard Topics: Hash Table, String, Sorting Solution Problem Understanding The problem asks us to find the longest self-contained substring in a given string s . A substring is self-contained if it does not share any characters with the rest of the string . More formally, for a substring t of s , every character in t must appear only inside t and not elsewhere in...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 412 - Fizz Buzz

LeetCode Problem 412 Difficulty: 🟢 Easy Topics: Math, String, Simulation Solution LeetCode 412, Fizz Buzz Problem Understanding The problem asks us to generate a list of strings for all integers from 1 to n . For each number, we apply a set of divisibility rules: If the number is divisible by both 3 and 5 , we append "FizzBuzz" . If the number is divisible only by 3 , we...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 187 - Repeated DNA Sequences

LeetCode Problem 187 Difficulty: 🟡 Medium Topics: Hash Table, String, Bit Manipulation, Sliding Window, Rolling Hash, Hash Function Solution Problem Understanding This problem asks us to find all repeated DNA subsequences of a fixed length, specifically length 10 , inside a given DNA string. The input is a string s , where each character represents a DNA nucleotide. The string contains only four possible characters: 'A' 'C' 'G' 'T' We...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1347 - Minimum Number of Steps to Make Two Strings Anagram

LeetCode Problem 1347 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding This problem asks us to determine the minimum number of character replacements needed to transform string t into an anagram of string s . Two strings are anagrams if they contain exactly the same characters with the same frequencies, regardless of order. Since both strings are guaranteed to have the same length, we never need to...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1366 - Rank Teams by Votes

LeetCode Problem 1366 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting, Counting Solution Problem Understanding The problem describes a voting based ranking system where every voter ranks all teams from best to worst. Each vote is represented as a string, where the character at index 0 is the top ranked team, the character at index 1 is the second ranked team, and so on. The final ranking is determined...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1781 - Sum of Beauty of All Substrings

LeetCode Problem 1781 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to calculate the sum of beauty for all substrings of a given string s . The beauty of a substring is defined as the difference between the highest frequency and the lowest frequency of any character that occurs in that substring. In simpler terms, for each substring, we count how many times...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 387 - First Unique Character in a String

LeetCode Problem 387 Difficulty: 🟢 Easy Topics: Hash Table, String, Queue, Counting Solution Problem Understanding The problem gives us a string s consisting only of lowercase English letters. We must find the index of the first character that appears exactly once in the entire string. If every character appears more than once, we return -1 . The key detail is that we are looking for the first non-repeating character based...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 843 - Guess the Word

LeetCode Problem 843 Difficulty: 🔴 Hard Topics: Array, Math, String, Interactive, Game Theory Solution LeetCode 843 - Guess the Word Problem Understanding This is an interactive problem where we must identify a hidden six-letter word from a given list of candidate words. We are not allowed to directly inspect the secret word. Instead, we can interact with the provided Master API by calling master.guess(word) . Each call returns the number...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1408 - String Matching in an Array

LeetCode Problem 1408 Difficulty: 🟢 Easy Topics: Array, String, String Matching Solution You included two different problems in one message, and the second problem (LeetCode 1408) appears to replace the first one while repeating the formatting instructions. I will proceed with LeetCode 1408 - String Matching in an Array . Problem Understanding The problem gives us an array of unique strings called words . We need to find every string...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 880 - Decoded String at Index

LeetCode Problem 880 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem gives us an encoded string s that represents a potentially enormous decoded string. The decoding process works incrementally from left to right. When we encounter a letter, we append it directly to the decoded tape. When we encounter a digit d , we repeat the entire current decoded tape d times in total. In other words,...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1790 - Check if One String Swap Can Make Strings Equal

LeetCode Problem 1790 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to determine if two strings of equal length, s1 and s2 , can be made identical by performing at most one string swap on exactly one of the strings . A string swap allows exchanging any two characters at different or same indices. The task is not to perform multiple swaps or...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2027 - Minimum Moves to Convert String

LeetCode Problem 2027 Difficulty: 🟢 Easy Topics: String, Greedy Solution Problem Understanding The problem asks us to transform a string s containing only characters 'X' and 'O' so that all characters become 'O' . A move consists of selecting three consecutive characters and converting them to 'O' . If a character is already 'O' , it remains unchanged. The goal is to determine the minimum number of moves required to...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1216 - Valid Palindrome III

LeetCode Problem 1216 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to determine whether a given string s can become a palindrome after removing at most k characters. A palindrome is a string that reads the same forwards and backwards, and a k -palindrome is one that can be turned into a palindrome with at most k deletions. The input consists of a string...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 824 - Goat Latin

LeetCode Problem 824 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding This problem asks us to transform a given sentence into a fictional language called Goat Latin , following a specific set of string manipulation rules. The input is a string called sentence , where words are separated by a single space. Every word contains only English alphabetic characters, meaning uppercase and lowercase letters. We must process each word individually...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1002 - Find Common Characters

LeetCode Problem 1002 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding The problem gives us an array of lowercase strings called words . We must return all characters that appear in every string in the array, including duplicate occurrences. The important detail is that duplicates matter. If a character appears multiple times in every word, then it must appear multiple times in the answer as well. For...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1405 - Longest Happy String

LeetCode Problem 1405 Difficulty: 🟡 Medium Topics: String, Greedy, Heap (Priority Queue) Solution Problem Understanding The problem asks us to construct the longest possible string using only the characters 'a' , 'b' , and 'c' , while satisfying a very important restriction: the string cannot contain three consecutive identical characters. In other words, substrings like "aaa" , "bbb" , and "ccc" are forbidden. We are given three integers: a ,...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 297 - Serialize and Deserialize Binary Tree

LeetCode Problem 297 Difficulty: 🔴 Hard Topics: String, Tree, Depth-First Search, Breadth-First Search, Design, Binary Tree Solution Problem Understanding The problem asks us to design two complementary operations for a binary tree: serialization and deserialization. Serialization converts a binary tree into a string representation that can be stored or transmitted. Deserialization performs the reverse operation, reconstructing the exact same binary tree structure from the serialized string. The important detail is...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 696 - Count Binary Substrings

LeetCode Problem 696 Difficulty: 🟢 Easy Topics: Two Pointers, String Solution Problem Understanding The problem gives us a binary string, meaning the string contains only the characters '0' and '1' . We need to count how many substrings satisfy two conditions simultaneously: The substring contains the same number of 0 s and 1 s. All 0 s are grouped together and all 1 s are grouped together. The second condition...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 420 - Strong Password Checker

LeetCode Problem 420 Difficulty: 🔴 Hard Topics: String, Greedy, Heap (Priority Queue) Solution Strong Password Checker Problem Understanding The problem asks us to determine the minimum number of operations required to transform a given password into a "strong" password according to three rules. A strong password must satisfy all of the following conditions: The password length must be between 6 and 20 characters inclusive. The password must contain at least...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 726 - Number of Atoms

LeetCode Problem 726 Difficulty: 🔴 Hard Topics: Hash Table, String, Stack, Sorting Solution Problem Understanding This problem asks us to parse a chemical formula represented as a string and return a canonical string that counts the number of each atom in the formula. Each atom starts with an uppercase letter and may have lowercase letters following it. A number following an atom represents how many of that atom are present,...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1147 - Longest Chunked Palindrome Decomposition

LeetCode Problem 1147 Difficulty: 🔴 Hard Topics: Two Pointers, String, Dynamic Programming, Greedy, Rolling Hash, Hash Function Solution Problem Understanding The problem asks us to decompose a given string text into the largest possible number of contiguous substrings such that the sequence of substrings forms a palindromic pattern. Specifically, the first substring must equal the last substring, the second substring must equal the second-to-last substring, and so on. The goal...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 383 - Ransom Note

LeetCode Problem 383 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding This problem asks whether it is possible to construct the string ransomNote using characters taken from the string magazine . Each character in magazine can only be used once, which means character frequency matters. If ransomNote requires more occurrences of a letter than magazine contains, the answer must be false . In other words, we need...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1078 - Occurrences After Bigram

LeetCode Problem 1078 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem gives us a string called text and two target words, first and second . We must find every occurrence where the words appear consecutively in the exact order: first second third For every such occurrence, we return the value of third . The important detail is that the words must appear immediately next to each other in...

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LeetCode 1061 - Lexicographically Smallest Equivalent String

LeetCode Problem 1061 Difficulty: 🟡 Medium Topics: String, Union-Find Solution Here is the complete, detailed technical solution guide for LeetCode 1061 following your exact formatting requirements. Problem Understanding The problem requires us to find the lexicographically smallest string that can be obtained from a given baseStr using character equivalencies defined by two strings, s1 and s2 . Each position i in s1 and s2 represents an equivalence relationship between s1[i]...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 409 - Longest Palindrome

LeetCode Problem 409 Difficulty: 🟢 Easy Topics: Hash Table, String, Greedy Solution Problem Understanding The problem asks us to find the length of the longest palindrome that can be constructed using the characters of a given string s . A palindrome is a string that reads the same forwards and backwards. The input string s contains only lowercase and uppercase English letters, and letter case matters, so 'A' and 'a'...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1684 - Count the Number of Consistent Strings

LeetCode Problem 1684 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Bit Manipulation, Counting Solution Problem Understanding This problem asks us to determine how many strings in the words array are consistent with a given set of allowed characters. You are given two inputs: allowed , a string containing distinct lowercase English letters. words , an array of lowercase strings. A word is considered consistent if every character in that...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 604 - Design Compressed String Iterator

LeetCode Problem 604 Difficulty: 🟢 Easy Topics: Array, String, Design, Iterator Solution Problem Understanding This problem asks us to implement a compressed string iterator . We are given a string where each character is immediately followed by a number representing how many times that character appears consecutively in the uncompressed version. For example, "a3b2" represents "aaabb" . We are required to implement a class StringIterator with two methods: next() ,...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1505 - Minimum Possible Integer After at Most K Adjacent Swaps On Digits

LeetCode Problem 1505 Difficulty: 🔴 Hard Topics: String, Greedy, Binary Indexed Tree, Segment Tree Solution Problem Understanding The problem gives us a numeric string num and an integer k . Each operation allows us to swap two adjacent digits. We may perform at most k such swaps, and the goal is to construct the smallest possible integer, returned as a string. The important detail is that swaps are only allowed...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 97 - Interleaving String

LeetCode Problem 97 Difficulty: 🟡 Medium Topics: String, Dynamic Programming Solution Problem Understanding This problem asks us to determine whether a string s3 can be formed by interleaving two other strings, s1 and s2 . An interleaving means we combine characters from s1 and s2 while preserving the relative order of characters within each original string . We are allowed to alternate between the two strings in any pattern, but...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1065 - Index Pairs of a String

LeetCode Problem 1065 Difficulty: 🟢 Easy Topics: Array, String, Trie, Sorting Solution Problem Understanding The problem gives us a string called text and a list of dictionary words called words . We must find every substring inside text that exactly matches one of the words in the dictionary. For every valid match, we return the pair [i, j] , where: i is the starting index of the substring j is...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 336 - Palindrome Pairs

LeetCode Problem 336 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Trie, Hash Function Solution Problem Understanding The problem gives us a list of unique strings called words . We must find every ordered pair of indices (i, j) such that: i != j concatenating words[i] + words[j] forms a palindrome A palindrome is a string that reads the same forward and backward. The important detail is that the pairs...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1433 - Check If a String Can Break Another String

LeetCode Problem 1433 Difficulty: 🟡 Medium Topics: String, Greedy, Sorting Solution Problem Understanding The problem asks us to determine whether one string can "break" another string through some permutation. Given two strings s1 and s2 of equal length, a string x can break string y if, after arranging both strings in some order, every character in x is greater than or equal to the corresponding character in y when compared...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 599 - Minimum Index Sum of Two Lists

LeetCode Problem 599 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding This problem is asking us to find the common elements between two lists of strings, but not just any common elements. We want the ones where the sum of their indices in both lists is the smallest. In other words, if a string appears at index i in list1 and index j in list2 , we...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1021 - Remove Outermost Parentheses

LeetCode Problem 1021 Difficulty: 🟢 Easy Topics: String, Stack Solution Unusual activity has been detected from your device. Try again later. (36c3723c-3fa3-4b63-a514-20d2635d5e47)

Practice › LeetCode › LeetCode 05xx ›

LeetCode 535 - Encode and Decode TinyURL

LeetCode Problem 535 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Hash Function Solution Problem Understanding The problem asks us to implement a URL shortening service, similar to TinyURL. Given a long URL, our system should generate a short, unique URL that maps back to the original URL. When the short URL is accessed, the system should return the original long URL. Essentially, we need a reversible mapping between long...

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LeetCode 1531 - String Compression II

LeetCode Problem 1531 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to minimize the length of a run-length encoded string after deleting at most k characters from the original string. Run-length encoding compresses consecutive repeated characters by writing the character followed by its count, but only when the count is greater than 1 . For example: "a" becomes "a" with length 1 "aa" becomes...

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LeetCode 1520 - Maximum Number of Non-Overlapping Substrings

LeetCode Problem 1520 Difficulty: 🔴 Hard Topics: Hash Table, String, Greedy, Sorting Solution LeetCode 1520 - Maximum Number of Non-Overlapping Substrings Problem Understanding This problem asks us to select as many non-overlapping substrings as possible from a given lowercase string s , while satisfying a strict containment rule. A substring is considered valid only if, whenever it contains a character c , it also contains every occurrence of c that...

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LeetCode 536 - Construct Binary Tree from String

LeetCode Problem 536 Difficulty: 🟡 Medium Topics: String, Stack, Tree, Depth-First Search, Binary Tree Solution Problem Understanding This problem asks us to reconstruct a binary tree from a specially formatted string representation. The input string contains integer values and parentheses. Every integer represents a tree node, and parentheses represent child subtrees. The structure follows a recursive pattern: A node value appears first. If the node has children, they are enclosed...

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LeetCode 1419 - Minimum Number of Frogs Croaking

LeetCode Problem 1419 Difficulty: 🟡 Medium Topics: String, Counting Solution Problem Understanding The problem asks us to determine the minimum number of frogs needed to produce a given sequence of croaks represented by the string croakOfFrogs . Each frog croaks in the sequence "c" -> "r" -> "o" -> "a" -> "k" , and multiple frogs can croak simultaneously. The challenge is that the letters from different frogs can interleave,...

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LeetCode 1106 - Parsing A Boolean Expression

LeetCode Problem 1106 Difficulty: 🔴 Hard Topics: String, Stack, Recursion Solution Problem Understanding The problem asks us to evaluate a boolean expression represented as a string . The expression can contain the literals 't' and 'f' for true and false, as well as three types of operators: logical NOT '!' , logical AND '&' , and logical OR '|' . Each operator has a specific syntax: '!(subExpr)' negates a sub-expression,...

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LeetCode 1830 - Minimum Number of Operations to Make String Sorted

LeetCode Problem 1830 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Combinatorics, Counting Solution Problem Understanding The operation described in the problem is exactly the process of generating the previous lexicographical permutation of a string. Starting from the current string, each operation transforms it into the lexicographically largest string that is still smaller than the current one. The goal is to determine how many such operations are required before the...

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LeetCode 1209 - Remove All Adjacent Duplicates in String II

LeetCode Problem 1209 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem asks us to repeatedly remove groups of exactly k adjacent identical characters from a string until no more such groups exist. A removal operation works as follows: Find k consecutive characters that are all the same. Remove those characters from the string. Concatenate the remaining left and right parts together. Continue checking again because new adjacent...

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LeetCode 1452 - People Whose List of Favorite Companies Is Not a Subset of Another List

LeetCode Problem 1452 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem requires identifying all people in a group whose list of favorite companies is not a subset of any other person's list . Each person has a list of companies they like, and we want the indices of people whose lists are unique in the sense that no other list completely contains theirs. The input...

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LeetCode 1249 - Minimum Remove to Make Valid Parentheses

LeetCode Problem 1249 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem is asking us to process a string containing lowercase letters and parentheses, and remove the minimum number of parentheses so that the remaining string is valid. A valid string is defined by the usual rules of parentheses: every opening parenthesis '(' must have a corresponding closing parenthesis ')' , and parentheses must be properly nested. The...

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LeetCode 921 - Minimum Add to Make Parentheses Valid

LeetCode Problem 921 Difficulty: 🟡 Medium Topics: String, Stack, Greedy Solution Problem Understanding The problem asks us to determine the minimum number of parentheses insertions required to make a given string of parentheses valid. A string of parentheses is considered valid if every opening parenthesis '(' has a matching closing parenthesis ')' and the pairs are properly nested. The input s is a string consisting only of '(' and ')'...

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LeetCode 115 - Distinct Subsequences

LeetCode Problem 115 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding This problem asks us to count how many distinct subsequences of a string s are equal to another string t . A subsequence is formed by deleting zero or more characters from a string without changing the relative order of the remaining characters . For example, "rabbit" is a subsequence of "rabbbit" because we can remove one...

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LeetCode 1003 - Check If Word Is Valid After Substitutions

LeetCode Problem 1003 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem asks us to determine whether a given string can be constructed by repeatedly inserting the substring "abc" into an initially empty string. The operation is very specific. At any point, we may take the current string and insert "abc" at any position. Because insertion can happen anywhere, valid strings may contain nested or interleaved patterns that...

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LeetCode 804 - Unique Morse Code Words

LeetCode Problem 804 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding The problem asks us to determine how many unique Morse code transformations exist among a list of words. Each lowercase English letter maps to a specific Morse code representation. A word can be transformed into Morse code by replacing each character with its Morse equivalent and concatenating the results together into a single string. For example,...

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LeetCode 2023 - Number of Pairs of Strings With Concatenation Equal to Target

LeetCode Problem 2023 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting Solution Problem Understanding The problem asks us to find the number of ordered pairs of indices (i, j) in a list of digit strings nums such that concatenating nums[i] and nums[j] produces exactly the string target . It is important that i and j are distinct , meaning the same element cannot pair with itself in the same...

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LeetCode 504 - Base 7

LeetCode Problem 504 Difficulty: 🟢 Easy Topics: Math, String Solution Problem Understanding The problem asks us to convert a given integer num into its representation in base 7, returning the result as a string. In other words, instead of representing the number in the usual decimal system (base 10), we represent it in a numeral system where each digit represents a power of 7, ranging from 0 to 6. For...

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LeetCode 248 - Strobogrammatic Number III

LeetCode Problem 248 Difficulty: 🔴 Hard Topics: Array, String, Recursion Solution Problem Understanding The problem asks us to count how many numbers within the inclusive range [low, high] are strobogrammatic. A strobogrammatic number is a number that appears unchanged when rotated 180 degrees. Only certain digits remain valid after rotation: Original Rotated 0 0 1 1 6 9 8 8 9 6 For example, "69" becomes "96" after rotation, but...

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LeetCode 1360 - Number of Days Between Two Dates

LeetCode Problem 1360 Difficulty: 🟢 Easy Topics: Math, String Solution Problem Understanding The problem gives two dates in the format YYYY-MM-DD and asks us to compute the absolute number of days between them. Each input string represents a valid calendar date. The year, month, and day are separated by hyphens. For example: "2019-06-29" means June 29, 2019 "2020-01-15" means January 15, 2020 The expected output is a single integer representing...

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LeetCode 937 - Reorder Data in Log Files

LeetCode Problem 937 Difficulty: 🟡 Medium Topics: Array, String, Sorting Solution Problem Understanding In this problem, we are given a list of log strings. Each log contains an identifier followed by one or more words separated by spaces. The first token is always the identifier, and everything after it represents the content of the log. The logs are divided into two categories: Letter-logs, where every word after the identifier contains...

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LeetCode 273 - Integer to English Words

LeetCode Problem 273 Difficulty: 🔴 Hard Topics: Math, String, Recursion Solution Problem Understanding The problem asks us to convert a non-negative integer into its English words representation. Instead of returning digits such as 12345 , we must produce a properly formatted English phrase such as "Twelve Thousand Three Hundred Forty Five" . The input is a single integer num where: 0 <= num <= 2^31 - 1 The maximum possible...

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LeetCode 1436 - Destination City

LeetCode Problem 1436 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding In this problem, we are given a list of directed paths between cities. Each entry in paths has the form [cityA, cityB] , which means there is a one way path from cityA to cityB . The graph formed by these paths is guaranteed to represent a straight travel chain with no cycles. This guarantee is...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1079 - Letter Tile Possibilities

LeetCode Problem 1079 Difficulty: 🟡 Medium Topics: Hash Table, String, Backtracking, Counting Solution Problem Understanding The problem asks us to count all possible non-empty sequences that can be formed from a set of letter tiles, where each tile has a single uppercase letter. The input is a string tiles representing the letters available on the tiles. The output is an integer representing the total number of distinct sequences that can...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 745 - Prefix and Suffix Search

LeetCode Problem 745 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Design, Trie Solution Problem Understanding The problem asks us to design a data structure that supports efficient searches based on both a prefix and a suffix. We are given an array of words, where each word has an implicit index based on its position in the array. The WordFilter class must support two operations: Initialization with the list of...

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LeetCode 467 - Unique Substrings in Wraparound String

LeetCode Problem 467 Difficulty: 🟡 Medium Topics: String, Dynamic Programming Solution Problem Understanding The problem defines an infinite cyclic string built from the lowercase English alphabet: "...zabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcd..." This means the sequence continues forever, and after 'z' comes 'a' again. Any substring that follows this cyclic ordering exists somewhere in the infinite wraparound string. We are given a string s , and we must determine how many distinct non-empty substrings of...

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LeetCode 784 - Letter Case Permutation

LeetCode Problem 784 Difficulty: 🟡 Medium Topics: String, Backtracking, Bit Manipulation Solution Problem Understanding The problem asks us to generate every possible string that can be formed by independently changing the case of each alphabetic character in the input string. Digits cannot be modified, so they always remain the same in every generated result. Only English letters can change between lowercase and uppercase. For example, if the input is "a1b2"...

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LeetCode 1047 - Remove All Adjacent Duplicates In String

LeetCode Problem 1047 Difficulty: 🟢 Easy Topics: String, Stack Solution Problem Understanding The problem gives us a string s containing only lowercase English letters. We repeatedly look for two adjacent characters that are equal, remove both of them, and continue this process until no such adjacent duplicate pair exists. The important detail is that removals can create new adjacent duplicates that were not adjacent before. For example, in the string...

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LeetCode 474 - Ones and Zeroes

LeetCode Problem 474 Difficulty: 🟡 Medium Topics: Array, String, Dynamic Programming Solution Problem Understanding This problem asks us to select the largest possible subset of binary strings while staying within two resource constraints. Each string consumes a certain number of 0 characters and a certain number of 1 characters. We are given two limits: m , the maximum number of zeros we can use n , the maximum number of...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1736 - Latest Time by Replacing Hidden Digits

LeetCode Problem 1736 Difficulty: 🟢 Easy Topics: String, Greedy Solution Problem Understanding The problem presents a string time formatted as hh:mm , where h and m are digits representing hours and minutes, respectively. Some digits may be hidden and are represented by a question mark ? . The goal is to replace the ? characters with digits such that the resulting time is the latest valid time possible , considering...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1328 - Break a Palindrome

LeetCode Problem 1328 Difficulty: 🟡 Medium Topics: String, Greedy Solution Problem Understanding The problem asks us to modify a given palindromic string in such a way that it is no longer a palindrome while ensuring that the resulting string is the lexicographically smallest possible. A palindrome is a string that reads the same forwards and backwards. In this problem, the input palindrome consists only of lowercase English letters and has...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1247 - Minimum Swaps to Make Strings Equal

LeetCode Problem 1247 Difficulty: 🟡 Medium Topics: Math, String, Greedy Solution Problem Understanding This problem asks us to determine the minimum number of swaps needed to make two strings, s1 and s2 , equal. Both strings are of the same length and consist only of the characters "x" and "y" . The allowed swap operation is restricted: you may only swap characters between the two strings, not within the same...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 751 - IP to CIDR

LeetCode Problem 751 Difficulty: 🟡 Medium Topics: String, Bit Manipulation Solution Problem Understanding This problem is asking us to take a starting IPv4 address and a number n of consecutive IP addresses and convert them into the minimal set of CIDR blocks that exactly covers that range. An IPv4 address is a 32-bit number written in the familiar dotted decimal format (e.g., "192.168.1.1" ), where each segment represents 8 bits....

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LeetCode 1062 - Longest Repeating Substring

LeetCode Problem 1062 Difficulty: 🟡 Medium Topics: String, Binary Search, Dynamic Programming, Rolling Hash, Suffix Array, Hash Function Solution Problem Understanding The problem asks us to find the length of the longest substring that appears at least twice in a given string s . A substring is a contiguous sequence of characters inside a string. The repeated substrings may overlap with each other, which is an important detail. For example,...

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LeetCode 127 - Word Ladder

LeetCode Problem 127 Difficulty: 🔴 Hard Topics: Hash Table, String, Breadth-First Search Solution Problem Understanding This problem asks us to determine the length of the shortest transformation sequence between two words, beginWord and endWord , under a strict transformation rule. A valid transformation changes exactly one character at a time , and every intermediate word must exist inside the provided wordList . The beginWord itself does not need to appear...

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LeetCode 290 - Word Pattern

LeetCode Problem 290 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem gives two inputs: a pattern string and a space-separated string of words. We need to determine whether the sequence of words follows the same structure as the sequence of characters in the pattern. A valid match requires a bijection between pattern characters and words. A bijection means the mapping works in both directions: Each character...

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LeetCode 1071 - Greatest Common Divisor of Strings

LeetCode Problem 1071 Difficulty: 🟢 Easy Topics: Math, String Solution Problem Understanding The problem asks us to find the greatest common divisor (GCD) of two strings. In this context, a string t divides another string s if s can be formed by concatenating t multiple times. The goal is to find the largest string x that divides both str1 and str2 . The input consists of two non-empty strings, str1...

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LeetCode 772 - Basic Calculator III

LeetCode Problem 772 Difficulty: 🔴 Hard Topics: Math, String, Stack, Recursion Solution LeetCode 772 - Basic Calculator III Problem Understanding This problem asks us to evaluate a mathematical expression represented as a string. The expression may contain: Non-negative integers Addition ( + ) Subtraction ( - ) Multiplication ( * ) Division ( / ) Parentheses ( ( and ) ) The goal is to compute the final integer result...

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LeetCode 686 - Repeated String Match

LeetCode Problem 686 Difficulty: 🟡 Medium Topics: String, String Matching Solution Problem Understanding The problem asks us to determine the minimum number of times a string a must be repeated such that another string b becomes a substring of the repeated version of a . A substring is a consecutive sequence of characters within a string. The input consists of two strings, a and b , both of length between...

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LeetCode 242 - Valid Anagram

LeetCode Problem 242 Difficulty: 🟢 Easy Topics: Hash Table, String, Sorting Solution Problem Understanding The problem asks us to determine whether two strings are anagrams of each other. Two strings are considered anagrams if they contain exactly the same characters with exactly the same frequencies , but possibly in a different order. In other words, one string can be rearranged to form the other. For example, "anagram" and "nagaram" are...

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LeetCode 902 - Numbers At Most N Given Digit Set

LeetCode Problem 902 Difficulty: 🔴 Hard Topics: Array, Math, String, Binary Search, Dynamic Programming Solution Problem Understanding The problem gives us a set of allowed digits, stored as strings, and an integer n . We may construct any positive integer by repeatedly using digits from the given set. Each digit can be reused any number of times. The goal is to count how many positive integers can be formed such...

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LeetCode 1668 - Maximum Repeating Substring

LeetCode Problem 1668 Difficulty: 🟢 Easy Topics: String, Dynamic Programming, String Matching Solution Problem Understanding This problem asks us to determine how many times a given string word can be repeated consecutively while still appearing as a substring inside another string called sequence . More formally, a string is considered k-repeating if word concatenated k times appears somewhere inside sequence as a contiguous substring. Our goal is to find the...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 884 - Uncommon Words from Two Sentences

LeetCode Problem 884 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution LeetCode 884, Uncommon Words from Two Sentences Problem Understanding This problem asks us to find all words that are considered "uncommon" between two sentences. Each sentence is made up of lowercase words separated by single spaces. A word is considered uncommon if it satisfies two conditions: It appears exactly once in one sentence. It does not appear at...

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LeetCode 1236 - Web Crawler

LeetCode Problem 1236 Difficulty: 🟡 Medium Topics: String, Depth-First Search, Breadth-First Search, Interactive Solution Problem Understanding The problem asks us to implement a simplified web crawler. We are given a starting URL and access to an HtmlParser interface that can retrieve all URLs linked from a given webpage. Our task is to traverse the web graph beginning from startUrl , but with two important restrictions: We may only visit URLs...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1545 - Find Kth Bit in Nth Binary String

LeetCode Problem 1545 Difficulty: 🟡 Medium Topics: String, Recursion, Simulation Solution Problem Understanding The problem defines a recursively constructed binary string sequence: S1 = "0" Si = Si-1 + "1" + reverse(invert(Si-1)) For every new level, we take the previous string, append "1" , then append the reversed and bit-inverted version of the previous string. The task is to return the kth bit in the string Sn . To understand...

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LeetCode 1544 - Make The String Great

LeetCode Problem 1544 Difficulty: 🟢 Easy Topics: String, Stack Solution Problem Understanding The problem asks us to process a string s consisting of lowercase and uppercase English letters, removing pairs of adjacent letters that are the same character but differ in case. Specifically, any pair where one character is lowercase and the adjacent character is its uppercase counterpart (or vice versa) is considered "bad." We must remove such pairs iteratively...

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LeetCode 1278 - Palindrome Partitioning III

LeetCode Problem 1278 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution LeetCode 1278 - Palindrome Partitioning III Problem Understanding The problem gives us a string s and an integer k . We are allowed to modify characters in the string, and our goal is to partition the string into exactly k non-empty substrings such that every substring becomes a palindrome after the modifications. A palindrome is a string that reads...

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LeetCode 1371 - Find the Longest Substring Containing Vowels in Even Counts

LeetCode Problem 1371 Difficulty: 🟡 Medium Topics: Hash Table, String, Bit Manipulation, Prefix Sum Solution LeetCode 1371 - Find the Longest Substring Containing Vowels in Even Counts Problem Understanding This problem asks us to find the length of the longest contiguous substring in which every vowel appears an even number of times. The vowels we care about are: a e i o u A vowel may appear zero times, and...

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LeetCode 551 - Student Attendance Record I

LeetCode Problem 551 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem gives us a string s where each character represents a student's attendance record for one day. There are only three possible characters: 'A' means the student was absent. 'L' means the student was late. 'P' means the student was present. We must determine whether the student qualifies for an attendance award. The student is eligible only if...

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LeetCode 514 - Freedom Trail

LeetCode Problem 514 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Depth-First Search, Breadth-First Search Solution LeetCode 514 - Freedom Trail Problem Understanding The problem models a circular dial, represented by the string ring , where each character is engraved at a position around the circle. Another string, key , represents the sequence of characters we must spell. At the beginning, the character at index 0 of ring is aligned at...

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LeetCode 72 - Edit Distance

LeetCode Problem 72 Difficulty: 🟡 Medium Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to compute the minimum number of edit operations needed to transform one string into another. The allowed operations are insertion, deletion, and replacement of a single character. Each operation has a cost of exactly one. We are given two input strings, word1 and word2 . The goal is to determine the smallest possible...

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LeetCode 1663 - Smallest String With A Given Numeric Value

LeetCode Problem 1663 Difficulty: 🟡 Medium Topics: String, Greedy Solution Problem Understanding The problem asks us to construct a lowercase string of length n such that the sum of the numeric values of its characters equals k , while also ensuring the resulting string is lexicographically smallest . Each lowercase letter has a numeric value equal to its position in the alphabet: a = 1 b = 2 c =...

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LeetCode 418 - Sentence Screen Fitting

LeetCode Problem 418 Difficulty: 🟡 Medium Topics: Array, String, Dynamic Programming Solution Problem Understanding The problem gives us a screen with a fixed number of rows and cols , along with a sentence represented as an array of words. We need to determine how many complete times the sentence can be written on the screen while following strict formatting rules. The sentence must be written left to right and top...

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LeetCode 28 - Find the Index of the First Occurrence in a String

LeetCode Problem 28 Difficulty: 🟢 Easy Topics: Two Pointers, String, String Matching Solution Problem Understanding This problem asks us to locate the first occurrence of one string inside another string. The string we are searching for is called needle , and the larger string we search inside is called haystack . More formally, we need to determine whether needle appears as a contiguous substring inside haystack . If it does,...

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LeetCode 1816 - Truncate Sentence

LeetCode Problem 1816 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding The problem gives us a sentence s and an integer k . A sentence is guaranteed to contain words separated by exactly one space, with no extra spaces at the beginning or end. Our task is to return a new sentence that contains only the first k words from the original sentence. In other words, we need to...

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LeetCode 1170 - Compare Strings by Frequency of the Smallest Character

LeetCode Problem 1170 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Binary Search, Sorting Solution Problem Understanding The problem defines a function f(s) for a non-empty string s . This function returns the frequency of the lexicographically smallest character in the string. For example: f("dcce") = 2 The smallest character is 'c' 'c' appears 2 times We are given two arrays: queries , containing query strings words , containing reference...

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LeetCode 499 - The Maze III

LeetCode Problem 499 Difficulty: 🔴 Hard Topics: Array, String, Depth-First Search, Breadth-First Search, Graph Theory, Heap (Priority Queue), Matrix, Shortest Path Solution LeetCode 499 - The Maze III Problem Understanding This problem extends the mechanics introduced in earlier Maze problems, but adds two important complications. First, we are no longer looking for a simple reachable or unreachable answer. Instead, we must find the shortest path by travel distance. Second, if...

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LeetCode 408 - Valid Word Abbreviation

LeetCode Problem 408 Difficulty: 🟢 Easy Topics: Two Pointers, String Solution Problem Understanding In this problem, we are given two strings: word , the original full word abbr , a supposed abbreviation of that word We must determine whether abbr is a valid abbreviation of word . An abbreviation works by replacing one or more non-empty substrings with their lengths. For example, "internationalization" can become "i12iz4n" because: "i" stays the...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 819 - Most Common Word

LeetCode Problem 819 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Counting Solution Problem Understanding The problem asks us to process a paragraph of text and determine which word appears most frequently, while ignoring a given list of banned words. The final answer must be returned in lowercase. Several important details make this problem slightly more involved than simple word counting. First, the paragraph may contain punctuation such as commas,...

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LeetCode 71 - Simplify Path

LeetCode Problem 71 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem gives us an absolute Unix-style file path and asks us to convert it into its canonical, simplified form. An absolute path always starts from the root directory, represented by / . The input path may contain unnecessary components such as repeated slashes, references to the current directory, or references to parent directories. Our job is to...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 320 - Generalized Abbreviation

LeetCode Problem 320 Difficulty: 🟡 Medium Topics: String, Backtracking, Bit Manipulation Solution LeetCode 320, Generalized Abbreviation Problem Understanding This problem asks us to generate every possible generalized abbreviation of a given word. A generalized abbreviation is created by replacing some characters in the string with their count, while preserving the order of the remaining characters. The important restriction is that abbreviated sections must be non-overlapping and non-adjacent. In practice, this...

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LeetCode 1556 - Thousand Separator

LeetCode Problem 1556 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to format a non-negative integer so that every group of three digits is separated by a dot ( . ), starting from the right side of the number. For example, the number 1234567 should become "1.234.567" because the digits are grouped as: 1 234 567 The input is a single integer n , and the...

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LeetCode 828 - Count Unique Characters of All Substrings of a Given String

LeetCode Problem 828 Difficulty: 🔴 Hard Topics: Hash Table, String, Dynamic Programming Solution Problem Understanding The problem asks us to compute the total number of unique characters across every possible substring of a given string. A character is considered unique inside a substring if it appears exactly once within that substring. For example, in the substring "ABA" , only "B" is unique because "A" appears twice. We are given a...

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LeetCode 1541 - Minimum Insertions to Balance a Parentheses String

LeetCode Problem 1541 Difficulty: 🟡 Medium Topics: String, Stack, Greedy Solution Problem Understanding This problem asks us to transform a string of parentheses into a special kind of balanced parentheses string using the minimum number of insertions. Unlike the standard parentheses balancing problem, this problem defines balance differently. Every opening parenthesis '(' must be matched with exactly two consecutive closing parentheses '))' . That means: One '(' requires two ')'...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 471 - Encode String with Shortest Length

LeetCode Problem 471 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Here’s a fully detailed technical solution guide following your requested format for LeetCode 471 - Encode String with Shortest Length . Problem Understanding The problem asks us to compress a string s into its shortest possible encoded form using the rule k[encoded_string] , where encoded_string is repeated exactly k times. The encoded form should only be used if it...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 591 - Tag Validator

LeetCode Problem 591 Difficulty: 🔴 Hard Topics: String, Stack Solution Problem Understanding This problem asks us to validate whether a given string represents a correctly structured code snippet according to a custom XML-like syntax. At first glance, it resembles parsing HTML or XML tags, but the validation rules are stricter and more specialized. The input is a single string code , which may contain opening tags, closing tags, CDATA sections,...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1358 - Number of Substrings Containing All Three Characters

LeetCode Problem 1358 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to count all substrings of a given string s that contain at least one occurrence of each character 'a' , 'b' , and 'c' . The string s is guaranteed to contain only these three characters. Essentially, we need to find the number of contiguous sequences in s where none of...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 301 - Remove Invalid Parentheses

LeetCode Problem 301 Difficulty: 🔴 Hard Topics: String, Backtracking, Breadth-First Search Solution Problem Understanding The problem asks us to remove the minimum number of invalid parentheses from a string so that the remaining string becomes valid. The input string may contain lowercase English letters in addition to parentheses. Letters do not affect validity and should always remain in the resulting strings unless they are removed as part of a broader...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1597 - Build Binary Expression Tree From Infix Expression

LeetCode Problem 1597 Difficulty: 🔴 Hard Topics: String, Stack, Tree, Binary Tree Solution Problem Understanding The problem asks us to construct a binary expression tree from a valid infix arithmetic expression string. An infix expression is the standard mathematical notation where operators appear between operands, such as 3+4 or 2*(5-1) . The input string may contain: Single digit operands Operators: + , - , * , / Parentheses ( and...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 161 - One Edit Distance

LeetCode Problem 161 Difficulty: 🟡 Medium Topics: Two Pointers, String Solution Problem Understanding The problem asks us to determine whether two strings are exactly one edit apart. An edit can be one of three operations: Insert a single character Delete a single character Replace one character with a different character The key detail is the word exactly. The strings must differ by precisely one valid edit. If they are already...

Practice › LeetCode › LeetCode 29xx ›

LeetCode 2981 - Find Longest Special Substring That Occurs Thrice I

LeetCode Problem 2981 Difficulty: 🟡 Medium Topics: Hash Table, String, Binary Search, Sliding Window, Counting Solution Problem Understanding The problem asks us to find the longest special substring that appears at least three times in the given string s . A substring is considered special if it consists entirely of the same character . This means strings like "aaa" , "bb" , and "z" are valid special substrings, while "ab"...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1055 - Shortest Way to Form String

LeetCode Problem 1055 Difficulty: 🟡 Medium Topics: Two Pointers, String, Binary Search, Greedy Solution Problem Understanding The problem asks us to construct the string target using the fewest possible subsequences of the string source . A subsequence preserves relative order, but characters do not need to be contiguous. For example, "ace" is a subsequence of "abcde" because we can delete "b" and "d" while keeping the order of the remaining...

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LeetCode 140 - Word Break II

LeetCode Problem 140 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Dynamic Programming, Backtracking, Trie, Memoization Solution Problem Understanding The problem asks us to segment a given string s into all possible sentences where each word in the sentence exists in a given dictionary wordDict . Essentially, we need to find all ways to insert spaces into the string such that every resulting substring is a valid dictionary word. The...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1048 - Longest String Chain

LeetCode Problem 1048 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, String, Dynamic Programming, Sorting Solution That is a long, structured technical guide that will exceed a practical single-message limit if done comprehensively with all requested sections, detailed walkthroughs, Python and Go solutions, worked examples, full test suite, and edge case analysis. I can provide it in full, following your exact formatting rules. I will include: Problem Understanding Brute...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1528 - Shuffle String

LeetCode Problem 1528 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding The problem asks us to take a string s and an integer array indices of the same length, and produce a new string where each character in s is moved to the position specified by indices . In other words, for each index i in s , the character s[i] should be placed at position indices[i] in the...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 609 - Find Duplicate File in System

LeetCode Problem 609 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem gives us a list of directory descriptions, where each string represents a directory and the files inside it. Each file entry includes both the filename and its content. For example: "root/a 1.txt(abcd) 2.txt(efgh)" means: The directory path is root/a Inside this directory: 1.txt contains "abcd" 2.txt contains "efgh" Our task is to identify files...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1422 - Maximum Score After Splitting a String

LeetCode Problem 1422 Difficulty: 🟢 Easy Topics: String, Prefix Sum Solution Problem Understanding The problem gives us a binary string s , meaning the string contains only the characters '0' and '1' . We must split this string into two non-empty parts, a left substring and a right substring. For every possible split position, we calculate a score defined as: the number of '0' characters in the left substring plus...

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LeetCode 243 - Shortest Word Distance

LeetCode Problem 243 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding The problem gives us an array of strings called wordsDict , along with two distinct target words, word1 and word2 . Both target words are guaranteed to exist somewhere in the array. Our goal is to compute the smallest distance between any occurrence of word1 and any occurrence of word2 . The distance between two words is defined...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 811 - Subdomain Visit Count

LeetCode Problem 811 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Counting Solution Problem Understanding In this problem, we are given a list of strings where each string represents a visit count paired with a domain name. Each input entry has the form: "count domain" For example: "9001 discuss.leetcode.com" means the domain discuss.leetcode.com was visited 9001 times. The important detail is that visiting a domain also counts as visiting all...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1678 - Goal Parser Interpretation

LeetCode Problem 1678 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding This problem asks us to interpret a Goal Parser command string . The string command consists of the characters "G" , "()" , and "(al)" in some order. Each symbol has a specific interpretation: "G" is interpreted as "G" , "()" is interpreted as "o" , and "(al)" is interpreted as "al" . The final result is the concatenation...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1081 - Smallest Subsequence of Distinct Characters

LeetCode Problem 1081 Difficulty: 🟡 Medium Topics: String, Stack, Greedy, Monotonic Stack Solution Problem Understanding The problem asks us to construct the lexicographically smallest subsequence of a string that contains every distinct character exactly once. A subsequence is formed by deleting zero or more characters without changing the relative order of the remaining characters. This is important because we are not allowed to rearrange characters arbitrarily. The final answer must...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1805 - Number of Different Integers in a String

LeetCode Problem 1805 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem requires us to process a string word that contains both lowercase English letters and digits. We are asked to extract all the sequences of digits as integers, ignoring any non-digit characters. After extracting these sequences, we need to count how many unique integers exist, considering that integers with leading zeros are equivalent to their non-zero...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 960 - Delete Columns to Make Sorted III

LeetCode Problem 960 Difficulty: 🔴 Hard Topics: Array, String, Dynamic Programming Solution Problem Understanding This problem asks us to find the minimum number of columns to delete from a set of equal-length strings so that each individual string becomes lexicographically sorted. In other words, we are allowed to remove some positions (columns) across all strings, and the goal is to ensure that each string, independently, is in non-decreasing alphabetical order...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 906 - Super Palindromes

LeetCode Problem 906 Difficulty: 🔴 Hard Topics: Math, String, Enumeration Solution LeetCode 906 - Super Palindromes Problem Understanding This problem asks us to count how many numbers within a given inclusive range are "super-palindromes". A number is considered a super-palindrome if both of the following conditions are true: The number itself is a palindrome. The number is the square of another palindrome. For example: 121 is a palindrome. 121 =...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 425 - Word Squares

LeetCode Problem 425 Difficulty: 🔴 Hard Topics: Array, String, Backtracking, Trie Solution Problem Understanding The problem is asking us to generate all possible word squares from a given list of unique words. A word square is a square arrangement of words such that the word at row i is identical to the word at column i for every i . In other words, reading across the rows and down the...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1177 - Can Make Palindrome from Substring

LeetCode Problem 1177 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Bit Manipulation, Prefix Sum Solution Problem Understanding The problem asks us to determine whether certain substrings of a given string s can be rearranged and partially modified to form a palindrome. A palindrome is a string that reads the same forward and backward. For each query [lefti, righti, ki] , we are allowed to take the substring s[lefti...righti] ,...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1163 - Last Substring in Lexicographical Order

LeetCode Problem 1163 Difficulty: 🔴 Hard Topics: Two Pointers, String Solution Problem Understanding The problem asks us to find the substring of a given string s that is lexicographically largest among all possible substrings. A substring is any contiguous portion of the string. For a string of length n , there are O(n²) possible substrings because every starting position can pair with many ending positions. Lexicographical order is the same...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 564 - Find the Closest Palindrome

LeetCode Problem 564 Difficulty: 🔴 Hard Topics: Math, String Solution Problem Understanding The problem gives us a string n representing a positive integer, and asks us to find the numerically closest palindrome that is not equal to the original number itself. A palindrome is a number that reads the same forward and backward. Examples include 121 , 999 , and 1331 . The key requirement is that we minimize the...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 664 - Strange Printer

LeetCode Problem 664 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem describes a "strange printer" that can print sequences of the same character in one turn. The printer can overwrite any existing characters in the string. Given an input string s , the goal is to determine the minimum number of printing operations required to print the entire string. In other words, we want to find...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 635 - Design Log Storage System

LeetCode Problem 635 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Ordered Set Solution Problem Understanding The problem is asking us to design a log storage system that supports two primary operations: storing logs with unique IDs and timestamps, and retrieving logs based on a timestamp range and granularity. Each timestamp is represented as a string in the format YYYY:MM:DD:HH:MM:SS , which is zero-padded and guarantees lexicographic ordering aligns with...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 93 - Restore IP Addresses

LeetCode Problem 93 Difficulty: 🟡 Medium Topics: String, Backtracking Solution Problem Understanding The problem asks us to take a string containing only digits and determine every possible way to insert exactly three dots so that the resulting string becomes a valid IPv4 address. A valid IP address has four numeric segments separated by dots. Each segment must satisfy two conditions: Its numeric value must be between 0 and 255 inclusive....

Practice › LeetCode › LeetCode 21xx ›

LeetCode 2120 - Execution of All Suffix Instructions Staying in a Grid

LeetCode Problem 2120 Difficulty: 🟡 Medium Topics: String, Simulation Solution Problem Understanding The problem asks us to simulate a robot moving on an n x n grid, starting from a specified position startPos = [startrow, startcol] . We are given a sequence of movement instructions as a string s , where each character represents a direction: 'L' for left, 'R' for right, 'U' for up, and 'D' for down. The...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1704 - Determine if String Halves Are Alike

LeetCode Problem 1704 Difficulty: 🟢 Easy Topics: String, Counting Solution Determine if String Halves Are Alike Problem Understanding The problem gives us a string s whose length is guaranteed to be even. We split the string into two equal parts: The first half is called a The second half is called b We must determine whether both halves contain the same number of vowels. The vowels include both lowercase and...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 500 - Keyboard Row

LeetCode Problem 500 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding This problem asks us to determine which words in a given list can be typed using letters from only one row of an American keyboard. The input is an array of strings, words , where each string represents a word consisting only of English alphabet characters. The task is to return only those words where every...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 592 - Fraction Addition and Subtraction

LeetCode Problem 592 Difficulty: 🟡 Medium Topics: Math, String, Simulation Solution Problem Understanding The problem requires computing the result of a mathematical expression containing fractions with addition and subtraction operators. The input is a single string, expression , which consists of positive or negative fractions in the format numerator/denominator . The output must be an irreducible fraction, represented as a string in the same format. If the result is an...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 943 - Find the Shortest Superstring

LeetCode Problem 943 Difficulty: 🔴 Hard Topics: Array, String, Dynamic Programming, Bit Manipulation, Bitmask Solution Problem Understanding The problem asks us to find the shortest superstring that contains all the given strings in the array words as substrings. In other words, we need a single string that includes every string in words at least once, concatenated in some order, but we want this superstring to have minimal length. The problem...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 482 - License Key Formatting

LeetCode Problem 482 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem gives us a string s that represents a license key. The string contains uppercase letters, lowercase letters, digits, and dashes. The existing dashes are only separators and do not necessarily represent the correct final grouping. We must reformat the string according to two rules: Every group after formatting must contain exactly k characters, except the first group....

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1316 - Distinct Echo Substrings

LeetCode Problem 1316 Difficulty: 🔴 Hard Topics: String, Trie, Rolling Hash, Hash Function Solution Problem Understanding The problem asks us to find the number of distinct non-empty substrings of a given string text that can be expressed as the concatenation of some string with itself. In other words, we need to count all substrings of the form a + a , where a is any non-empty string. For example, "abcabc"...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 936 - Stamping The Sequence

LeetCode Problem 936 Difficulty: 🔴 Hard Topics: String, Stack, Greedy, Queue Solution Problem Understanding This problem asks us to reconstruct a target string by repeatedly applying a smaller string called stamp . We begin with a string s that has the same length as target , but every character is initially '?' . In one operation, we may place the stamp string onto any valid position in s , and...

Practice › LeetCode › LeetCode 30xx ›

LeetCode 3043 - Find the Length of the Longest Common Prefix

LeetCode Problem 3043 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie Solution Problem Understanding We are given two arrays of positive integers , arr1 and arr2 . The task is to determine the maximum length of a common digit prefix between any pair of numbers (x, y) where x comes from arr1 and y comes from arr2 . A prefix of a number means taking digits from the beginning,...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 171 - Excel Sheet Column Number

LeetCode Problem 171 Difficulty: 🟢 Easy Topics: Math, String Solution Problem Understanding This problem asks us to convert an Excel-style column title into its corresponding numerical index. In Excel spreadsheets, columns are labeled alphabetically: A -> 1 B -> 2 ... Z -> 26 AA -> 27 AB -> 28 The labeling system works similarly to a positional number system, except instead of digits 0-9 , it uses letters A-Z...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1320 - Minimum Distance to Type a Word Using Two Fingers

LeetCode Problem 1320 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The keyboard is arranged in a fixed grid layout containing the 26 uppercase English letters. Each letter has a coordinate in the grid: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z The coordinates are assigned row by row. For...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 727 - Minimum Window Subsequence

LeetCode Problem 727 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Sliding Window Solution Problem Understanding This problem asks us to find the smallest contiguous substring of s1 such that s2 appears inside that substring as a subsequence. A subsequence does not require characters to be adjacent, but they must appear in the same relative order. For example, "bde" is a subsequence of "bcde" because we can pick 'b' , then...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1689 - Partitioning Into Minimum Number Of Deci-Binary Numbers

LeetCode Problem 1689 Difficulty: 🟡 Medium Topics: String, Greedy Solution Problem Understanding The problem asks us to determine the minimum number of positive deci-binary numbers required to sum up to a given decimal number represented as a string. A deci-binary number is a number whose digits consist only of 0 and 1 , and it cannot contain leading zeros. For example: 101 is valid because every digit is either 0...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1796 - Second Largest Digit in a String

LeetCode Problem 1796 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem gives us a string s that contains lowercase English letters and numerical digits. Our task is to find the second largest distinct digit that appears anywhere in the string. The important detail is that we care about distinct digits, not frequency. If the string contains "111222333" , the distinct digits are {1, 2, 3} ....

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1842 - Next Palindrome Using Same Digits

LeetCode Problem 1842 Difficulty: 🔴 Hard Topics: Two Pointers, String Solution Problem Understanding The input is a numeric string num that is guaranteed to already be a palindrome. The task is to rearrange its digits to create another palindrome that is strictly larger than the original number, while also being the smallest such palindrome possible. A palindrome has mirror symmetry. For example: "1221" is a palindrome because it reads the...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 151 - Reverse Words in a String

LeetCode Problem 151 Difficulty: 🟡 Medium Topics: Two Pointers, String Solution Problem Understanding The problem asks us to take a string s containing words separated by spaces and return a new string where the words appear in reverse order . A word is defined as any sequence of non-space characters, and words can be separated by multiple spaces. The output must normalize spaces so that only a single space exists...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1180 - Count Substrings with Only One Distinct Letter

LeetCode Problem 1180 Difficulty: 🟢 Easy Topics: Math, String Solution Problem Understanding The problem asks us to count how many substrings of a given string contain only one distinct character. A substring is a contiguous section of the string. For example, in the string "aaaba" , the substring "aaa" is valid because every character is 'a' . The substring "aab" is not valid because it contains both 'a' and 'b'...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1397 - Find All Good Strings

LeetCode Problem 1397 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, String Matching Solution Problem Understanding The problem asks us to count how many strings of length n satisfy three conditions simultaneously: The string must be lexicographically greater than or equal to s1 The string must be lexicographically less than or equal to s2 The string must not contain evil as a substring anywhere The result can become extremely large, so...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1234 - Replace the Substring for Balanced String

LeetCode Problem 1234 Difficulty: 🟡 Medium Topics: String, Sliding Window Solution Problem Understanding The problem asks us to balance a string composed of exactly four types of characters: 'Q' , 'W' , 'E' , and 'R' . A string is balanced when each character occurs exactly n / 4 times, where n is the length of the string. The input is a string s of length n (guaranteed to be...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1585 - Check If String Is Transformable With Substring Sort Operations

LeetCode Problem 1585 Difficulty: 🔴 Hard Topics: String, Greedy, Sorting Solution Problem Understanding This problem asks whether we can transform one digit string, s , into another digit string, t , using a special operation. In one operation, we choose any non-empty contiguous substring of s and sort that substring in ascending order. The important detail is that sorting a substring only moves smaller digits toward the left and larger...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1119 - Remove Vowels from a String

LeetCode Problem 1119 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to remove vowels from a given string s . Specifically, the vowels are defined as the lowercase characters 'a' , 'e' , 'i' , 'o' , and 'u' . The input string consists only of lowercase English letters, and its length ranges from 1 to 1000. The expected output is a string that contains all...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1772 - Sort Features by Popularity

LeetCode Problem 1772 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding This problem asks us to determine the popularity of product features based on user survey responses. We are given a list of features where each element is a single-word feature name, and a list of responses where each element is a string of space-separated words that users reported liking. The popularity of a feature is...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1309 - Decrypt String from Alphabet to Integer Mapping

LeetCode Problem 1309 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem gives us a string containing digits and the '#' character. This string encodes lowercase English letters using a special mapping rule. The mapping works in two different ways: Digits '1' through '9' represent the letters 'a' through 'i' Two digit numbers followed by '#' , from "10#" through "26#" , represent the letters 'j' through 'z' Our...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 179 - Largest Number

LeetCode Problem 179 Difficulty: 🟡 Medium Topics: Array, String, Greedy, Sorting Solution LeetCode 179 - Largest Number Problem Understanding This problem asks us to rearrange a list of non-negative integers so that, when concatenated together, they form the largest possible number. At first glance, this may look like a straightforward sorting problem where we simply sort the numbers in descending order. However, the challenge comes from the fact that the...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 139 - Word Break

LeetCode Problem 139 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Dynamic Programming, Trie, Memoization Solution Problem Understanding The problem asks us to determine whether a given string s can be broken into a sequence of valid words from a given dictionary wordDict . Essentially, we need to check if we can insert spaces into s such that every substring separated by these spaces exists in the dictionary. The input...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 438 - Find All Anagrams in a String

LeetCode Problem 438 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem gives two strings, s and p , both containing only lowercase English letters. We need to find every starting index in s where a substring is an anagram of p . An anagram means the substring contains exactly the same characters as p , with the same frequencies, but possibly in a different...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 358 - Rearrange String k Distance Apart

LeetCode Problem 358 Difficulty: 🔴 Hard Topics: Hash Table, String, Greedy, Sorting, Heap (Priority Queue), Counting Solution LeetCode 358 - Rearrange String k Distance Apart Problem Understanding The problem gives us a string s and an integer k . We must rearrange the characters in the string so that identical characters are separated by at least k positions. The phrase "at least distance k " means that if the same...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 676 - Implement Magic Dictionary

LeetCode Problem 676 Difficulty: 🟡 Medium Topics: Hash Table, String, Depth-First Search, Design, Trie Solution Problem Understanding The problem asks us to design a dictionary-like data structure that supports two operations. First, we load a collection of distinct words into the structure. Second, given a query string, we determine whether it is possible to modify exactly one character in the query so that the resulting string matches one of the...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1239 - Maximum Length of a Concatenated String with Unique Characters

LeetCode Problem 1239 Difficulty: 🟡 Medium Topics: Array, String, Backtracking, Bit Manipulation Solution Problem Understanding This problem asks us to select a subsequence of strings from the input array arr and concatenate them together such that the resulting string contains only unique characters. Among all valid concatenations, we must return the maximum possible length. A subsequence means we can either include or skip each string while preserving the original order....

Practice › LeetCode › LeetCode 07xx ›

LeetCode 712 - Minimum ASCII Delete Sum for Two Strings

LeetCode Problem 712 Difficulty: 🟡 Medium Topics: String, Dynamic Programming Solution Problem Understanding The problem gives us two strings, s1 and s2 , and asks us to make them equal by deleting characters from either string. Every deleted character contributes its ASCII value to the total cost. Our goal is to minimize this total deletion cost. For example, if we delete the character 's' , we add 115 to the...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1745 - Palindrome Partitioning IV

LeetCode Problem 1745 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to determine whether a given string s can be split into exactly three non-empty substrings , each of which is a palindrome. A palindrome is a string that reads the same forward and backward. For example, the string "aba" is a palindrome, whereas "abc" is not. The input s is guaranteed to have...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 459 - Repeated Substring Pattern

LeetCode Problem 459 Difficulty: 🟢 Easy Topics: String, String Matching Solution Problem Understanding The problem asks us to determine whether a given string can be formed by repeating one of its substrings multiple times. In other words, we want to know if there exists some substring pattern such that concatenating that pattern repeatedly recreates the entire string exactly. For example, consider the string "abab" : The substring "ab" repeated twice...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1812 - Determine Color of a Chessboard Square

LeetCode Problem 1812 Difficulty: 🟢 Easy Topics: Math, String Solution LeetCode 1812 - Determine Color of a Chessboard Square Problem Understanding This problem asks us to determine whether a given chessboard square is white or black based on its coordinate. The input is a string of length two, where the first character is a lowercase letter from 'a' to 'h' , representing the column, and the second character is a...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 647 - Palindromic Substrings

LeetCode Problem 647 Difficulty: 🟡 Medium Topics: Two Pointers, String, Dynamic Programming Solution Problem Understanding The problem asks us to count how many palindromic substrings exist inside a given string s . A substring is any contiguous segment of the string. This means we cannot rearrange characters or skip positions. For example, in "abc" , "ab" is a substring, but "ac" is not because the characters are not adjacent. A...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1202 - Smallest String With Swaps

LeetCode Problem 1202 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Depth-First Search, Breadth-First Search, Union-Find, Sorting Solution Problem Understanding The problem gives us a string s and a list of index pairs called pairs . Each pair [a, b] means we are allowed to swap the characters at positions a and b . The important detail is that swaps can be performed any number of times. This changes the...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 211 - Design Add and Search Words Data Structure

LeetCode Problem 211 Difficulty: 🟡 Medium Topics: String, Depth-First Search, Design, Trie Solution Problem Understanding The problem asks us to design a data structure that supports two operations efficiently: Adding words into the structure. Searching for words, where the search pattern may contain the wildcard character '.' . The wildcard '.' represents any single lowercase English letter. This means that a search query is not always an exact string match....

Practice › LeetCode › LeetCode 05xx ›

LeetCode 557 - Reverse Words in a String III

LeetCode Problem 557 Difficulty: 🟢 Easy Topics: Two Pointers, String Solution Problem Understanding The problem requires reversing the characters of each individual word in a string while maintaining the original order of the words and the spacing between them. In other words, the sentence structure remains the same, but each word is mirrored in place. For example, the word "Let's" becomes "s'teL" , and "LeetCode" becomes "edoCteeL" . The input...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 649 - Dota2 Senate

LeetCode Problem 649 Difficulty: 🟡 Medium Topics: String, Greedy, Queue Solution Problem Understanding The problem describes a turn-based voting process between two parties in the Dota2 senate, the Radiant party represented by 'R' and the Dire party represented by 'D' . We are given a string senate where each character represents a senator and their party affiliation. The order of characters in the string is important because senators act sequentially...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1233 - Remove Sub-Folders from the Filesystem

LeetCode Problem 1233 Difficulty: 🟡 Medium Topics: Array, String, Depth-First Search, Trie Solution Problem Understanding This problem asks us to process a list of folder paths and remove all sub-folders , returning only the top-level folders. A folder "/a/b" is considered a sub-folder of "/a" because it is nested inside it. The key detail is that a sub-folder must start with the parent folder path, followed by a '/' ....

Practice › LeetCode › LeetCode 08xx ›

LeetCode 800 - Similar RGB Color

LeetCode Problem 800 Difficulty: 🟢 Easy Topics: Math, String, Enumeration Solution Problem Understanding The problem is asking us to find the shorthand RGB color that is most similar to a given full-length RGB color string. An RGB color is represented as a hexadecimal string of the form "#RRGGBB" , where each pair of characters ( RR , GG , BB ) represents the red, green, and blue components, respectively, using...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 186 - Reverse Words in a String II

LeetCode Problem 186 Difficulty: 🟡 Medium Topics: Two Pointers, String Solution Problem Understanding The problem asks us to reverse the order of words in a character array s in-place . Each word is a contiguous sequence of non-space characters, and words are separated by exactly one space. The input array does not have leading or trailing spaces, so we do not need to handle extra spaces at the ends. The...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 678 - Valid Parenthesis String

LeetCode Problem 678 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Stack, Greedy Solution Problem Understanding This problem is asking whether a string containing the characters '(' , ')' , and '*' can be considered a valid parenthesis string. In essence, '(' must be matched by a ')' in the correct order, and '*' can act as a flexible placeholder that behaves as '(' , ')' , or an empty string....

Practice › LeetCode › LeetCode 05xx ›

LeetCode 527 - Word Abbreviation

LeetCode Problem 527 Difficulty: 🔴 Hard Topics: Array, String, Greedy, Trie, Sorting Solution Problem Understanding The problem asks us to compute minimal unique abbreviations for an array of distinct strings. An abbreviation replaces the middle characters of a word with a count, keeping the first and last characters. For example, "internal" becomes "i6l" because there are six letters between i and l . The challenge arises when multiple words share...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 544 - Output Contest Matches

LeetCode Problem 544 Difficulty: 🟡 Medium Topics: String, Recursion, Simulation Solution Problem Understanding The problem asks us to simulate the structure of a playoff tournament bracket. We are given n teams, numbered from 1 to n , where smaller numbers represent stronger teams. The tournament pairing rule is fixed: in every round, the strongest remaining team must play against the weakest remaining team. The output is not the winner of...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1784 - Check if Binary String Has at Most One Segment of Ones

LeetCode Problem 1784 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding In this problem, we are given a binary string s that contains only the characters '0' and '1' . The string is guaranteed to begin with '1' , which means there are no leading zeros. The task is to determine whether the string contains at most one contiguous segment of ones. A contiguous segment of ones means a sequence...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 488 - Zuma Game

LeetCode Problem 488 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Stack, Breadth-First Search, Memoization Solution Problem Understanding The problem models a recursive elimination game played on a row of colored balls. The board is represented as a string where each character corresponds to a colored ball. The available colors are 'R' , 'Y' , 'B' , 'G' , and 'W' . You are also given another string called hand ,...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1662 - Check If Two String Arrays are Equivalent

LeetCode Problem 1662 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding The problem gives us two arrays of strings, word1 and word2 . Each array represents a single larger string formed by concatenating all of its elements in order. Our task is to determine whether the final strings produced by both arrays are exactly the same. For example, if: word1 = ["ab", "c"] word2 = ["a", "bc"] then: word1...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2002 - Maximum Product of the Length of Two Palindromic Subsequences

LeetCode Problem 2002 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Backtracking, Bit Manipulation, Bitmask Solution Problem Understanding The problem is asking us to find two disjoint palindromic subsequences from a given string s such that the product of their lengths is maximized. A subsequence is derived by deleting zero or more characters from the original string while keeping the relative order intact. A palindrome reads the same forward and backward,...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2024 - Maximize the Confusion of an Exam

LeetCode Problem 2024 Difficulty: 🟡 Medium Topics: String, Binary Search, Sliding Window, Prefix Sum Solution Problem Understanding This problem asks us to maximize the length of a contiguous block of identical answers in a true/false exam answer key. The input string answerKey contains only two characters, 'T' and 'F' , representing the answers for each question. We are also given an integer k , which represents the maximum number of...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 434 - Number of Segments in a String

LeetCode Problem 434 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to count how many separate word-like groups exist inside a string. In this problem, a "segment" is defined as a continuous sequence of characters that are not spaces. Every time we encounter a block of non-space characters separated by spaces, that block counts as one segment. For example, in the string "Hello, my name is...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 76 - Minimum Window Substring

LeetCode Problem 76 Difficulty: 🔴 Hard Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to find the smallest contiguous substring inside string s that contains every character from string t , including duplicate occurrences. A substring must consist of consecutive characters. We are not allowed to reorder characters or skip positions. The window must contain every required character with at least the same frequency as...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 244 - Shortest Word Distance II

LeetCode Problem 244 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, String, Design Solution LeetCode 244 - Shortest Word Distance II Problem Understanding This problem asks us to design a reusable data structure that can efficiently answer repeated shortest-distance queries between words in a fixed list of strings. We are given an array of words, wordsDict , during initialization. After that, we must repeatedly answer queries asking: What is...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 737 - Sentence Similarity II

LeetCode Problem 737 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Depth-First Search, Breadth-First Search, Union-Find Solution Problem Understanding This problem asks us to determine whether two sentences are considered similar based on a set of similarity relationships between words. Each sentence is represented as an array of strings, where each string is a single word. We are also given a list of word pairs, where each pair indicates that...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 266 - Palindrome Permutation

LeetCode Problem 266 Difficulty: 🟢 Easy Topics: Hash Table, String, Bit Manipulation Solution Problem Understanding The problem asks whether the characters of a given string can be rearranged to form a palindrome. A palindrome is a string that reads the same forward and backward. Examples include "racecar" , "abba" , and "a" . The important detail is that we are not asked to actually generate the palindrome. We only need...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 792 - Number of Matching Subsequences

LeetCode Problem 792 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Binary Search, Dynamic Programming, Trie, Sorting Solution Problem Understanding The problem asks us to count how many words in a given list are subsequences of a string s . A subsequence is formed by deleting zero or more characters from the string without changing the order of the remaining characters. For instance, "ace" is a subsequence of "abcde" because...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1143 - Longest Common Subsequence

LeetCode Problem 1143 Difficulty: 🟡 Medium Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to find the length of the longest subsequence that appears in both input strings. A subsequence is formed by deleting some characters from a string while keeping the remaining characters in the same relative order. The characters do not need to be contiguous. For example, in the string "abcde" : "ace" is a...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1208 - Get Equal Substrings Within Budget

LeetCode Problem 1208 Difficulty: 🟡 Medium Topics: String, Binary Search, Sliding Window, Prefix Sum Solution Problem Understanding The problem asks us to determine the longest contiguous substring of s that can be transformed into the corresponding substring of t without exceeding a given budget, maxCost . For each character in the substring, the transformation cost is the absolute difference between the ASCII values of the characters in s and t...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2067 - Number of Equal Count Substrings

LeetCode Problem 2067 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window, Counting Solution Problem Understanding The problem asks us to count the number of substrings in a string s such that every unique character in the substring occurs exactly count times . A substring is a contiguous segment of the string, so we are only considering consecutive characters. Inputs are: s : a string of lowercase English letters, length...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 539 - Minimum Time Difference

LeetCode Problem 539 Difficulty: 🟡 Medium Topics: Array, Math, String, Sorting Solution Problem Understanding The problem gives a list of time points in 24-hour clock format , where every time is represented as a string in the form "HH:MM" . Your task is to determine the smallest difference in minutes between any two time points in the list. A key detail is that time is cyclic , meaning the clock...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 820 - Short Encoding of Words

LeetCode Problem 820 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie Solution Problem Understanding The problem asks us to find the length of the shortest reference string that can encode a list of words. A reference string is formed by concatenating some of the words with a '#' character at the end of each word. Importantly, a word can be represented by a substring of the reference string if...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1087 - Brace Expansion

LeetCode Problem 1087 Difficulty: 🟡 Medium Topics: String, Backtracking, Stack, Breadth-First Search, Sorting Solution Problem Understanding In this problem, we are given a string that represents multiple possible words. Each position in the word may contain either a single fixed character or a set of alternative characters enclosed in curly braces. For example: a{b,c}d means: The first character is always 'a' The second character can be either 'b' or 'c'...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1487 - Making File Names Unique

LeetCode Problem 1487 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem asks us to simulate how a file system assigns folder names when duplicate names appear. We are given an array names , where names[i] represents the folder name requested at the i-th minute. The file system must ensure that every created folder has a unique name. If a requested name has never been used...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1769 - Minimum Number of Operations to Move All Balls to Each Box

LeetCode Problem 1769 Difficulty: 🟡 Medium Topics: Array, String, Prefix Sum Solution Problem Understanding This problem asks us to compute the minimum number of moves required to gather all balls in a string of boxes into each individual box. Each box can either be empty ( '0' ) or contain one ball ( '1' ). A single operation consists of moving a ball from one box to an adjacent box....

Practice › LeetCode › LeetCode 00xx ›

LeetCode 30 - Substring with Concatenation of All Words

LeetCode Problem 30 Difficulty: 🔴 Hard Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to find all starting indices in a string s where a substring exists that is a concatenation of all the words in the array words exactly once, without any intervening characters. The words can appear in any order, and all words in words have the same length. The input s is...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 681 - Next Closest Time

LeetCode Problem 681 Difficulty: 🟡 Medium Topics: Hash Table, String, Backtracking, Enumeration Solution Problem Understanding The problem gives us a valid time string in the format "HH:MM" and asks us to construct the next chronological time using only the digits already present in the original time. The important detail is that digits may be reused any number of times. If the input is "19:34" , the allowed digits are {1,...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 299 - Bulls and Cows

LeetCode Problem 299 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting Solution Problem Understanding The problem gives two strings, secret and guess , representing two numbers of equal length. We need to compare them and return a hint in the format "xAyB" . A "bull" is a digit that matches in both value and position. For example, if secret[i] == guess[i] , then that digit contributes one bull. A "cow"...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 316 - Remove Duplicate Letters

LeetCode Problem 316 Difficulty: 🟡 Medium Topics: String, Stack, Greedy, Monotonic Stack Solution LeetCode 316, Remove Duplicate Letters Problem Understanding This problem asks us to construct a new string from the input string s such that every distinct character appears exactly once, while also ensuring that the final result is the smallest possible in lexicographical order. The input is a string consisting only of lowercase English letters. Characters may appear...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 642 - Design Search Autocomplete System

LeetCode Problem 642 Difficulty: 🔴 Hard Topics: String, Depth-First Search, Design, Trie, Sorting, Heap (Priority Queue), Data Stream Solution Problem Understanding The problem asks us to design an autocomplete system that behaves similarly to a search engine suggestion feature. The system starts with a collection of historical sentences and the number of times each sentence has been typed before. As the user types characters one by one, the system must...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1032 - Stream of Characters

LeetCode Problem 1032 Difficulty: 🔴 Hard Topics: Array, String, Design, Trie, Data Stream Solution Problem Understanding The problem asks us to design a data structure that processes a stream of characters one at a time. After each newly added character, we must determine whether any suffix of the stream matches one of the words from a predefined dictionary. A suffix means the ending portion of the stream. For example, if...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1348 - Tweet Counts Per Frequency

LeetCode Problem 1348 Difficulty: 🟡 Medium Topics: Hash Table, String, Binary Search, Design, Sorting, Ordered Set Solution Problem Understanding The problem is asking us to design a system that tracks tweets by their timestamp and allows querying the number of tweets in specific time intervals, split according to a frequency (minute, hour, or day). Specifically, we need to implement a TweetCounts class with two operations: recording a tweet and retrieving...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 758 - Bold Words in String

LeetCode Problem 758 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie, String Matching Solution Problem Understanding The problem gives us two inputs: an array of strings called words , and a target string s . Every occurrence of every word from words inside s must become bold by surrounding that substring with and tags. The important requirement is that we must use the minimum number of tags...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 988 - Smallest String Starting From Leaf

LeetCode Problem 988 Difficulty: 🟡 Medium Topics: String, Backtracking, Tree, Depth-First Search, Binary Tree Solution Problem Understanding The problem gives us the root of a binary tree where every node stores an integer between 0 and 25 . Each integer corresponds to a lowercase English letter: 0 -> 'a' 1 -> 'b' ... 25 -> 'z' We need to construct strings that begin at a leaf node and move upward...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 131 - Palindrome Partitioning

LeetCode Problem 131 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Backtracking Solution LeetCode 131, Palindrome Partitioning Problem Understanding The problem gives us a string s and asks us to divide it into substrings such that every substring is a palindrome. A palindrome is a string that reads the same forward and backward. We must return all possible valid ways to partition the string. A partition means splitting the string into...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 484 - Find Permutation

LeetCode Problem 484 Difficulty: 🟡 Medium Topics: Array, String, Stack, Greedy Solution Problem Understanding The problem gives us a string s consisting only of the characters 'I' and 'D' . This string describes the relationship between adjacent numbers in a hidden permutation. If s[i] == 'I' , then the permutation must satisfy: perm[i] < perm[i + 1] If s[i] == 'D' , then the permutation must satisfy: perm[i] > perm[i...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 246 - Strobogrammatic Number

LeetCode Problem 246 Difficulty: 🟢 Easy Topics: Hash Table, Two Pointers, String Solution Problem Understanding The problem asks us to determine whether a given numeric string remains valid and unchanged when rotated 180 degrees. This property is called being "strobogrammatic". A strobogrammatic number is formed only by digits that still represent valid digits after rotation. The valid transformations are: 0 -> 0 1 -> 1 6 -> 9 8 ->...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1221 - Split a String in Balanced Strings

LeetCode Problem 1221 Difficulty: 🟢 Easy Topics: String, Greedy, Counting Solution LeetCode 1221 - Split a String in Balanced Strings Problem Understanding This problem gives us a string s that contains only the characters 'L' and 'R' . The string is guaranteed to already be balanced overall, meaning the total number of 'L' characters equals the total number of 'R' characters. Our task is to split this string into the...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 944 - Delete Columns to Make Sorted

LeetCode Problem 944 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding The problem gives us an array of strings where every string has exactly the same length. If we place all strings vertically, character by character, the strings form a rectangular grid. For example, given: ["abc", "bce", "cae"] the grid becomes: abc bce cae Each column is formed by taking characters from the same index across all rows. The...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 65 - Valid Number

LeetCode Problem 65 Difficulty: 🔴 Hard Topics: String Solution This is a comprehensive, multi section technical guide that will be quite long if completed properly with all requested sections, detailed explanations, two implementations, walkthroughs, worked examples, complexity analysis, exhaustive test cases, and edge case discussions. To keep quality high and avoid truncation, I will provide it in a structured document format.

Practice › LeetCode › LeetCode 02xx ›

LeetCode 271 - Encode and Decode Strings

LeetCode Problem 271 Difficulty: 🟡 Medium Topics: Array, String, Design Solution Problem Understanding The problem asks us to design a reversible encoding system for a list of strings. We need two functions: encode , which converts a list of strings into a single string decode , which reconstructs the original list from that encoded string The important requirement is that the transformation must be lossless. After encoding and then decoding,...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 722 - Remove Comments

LeetCode Problem 722 Difficulty: 🟡 Medium Topics: Array, String Solution Problem Understanding The problem asks us to remove comments from a C++ program represented as an array of strings. Each string represents a line of the program, split by newline characters. In C++, there are two types of comments: line comments starting with "//" and block comments enclosed between "/*" and "*/" . Line comments terminate at the end of...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1041 - Robot Bounded In Circle

LeetCode Problem 1041 Difficulty: 🟡 Medium Topics: Math, String, Simulation Solution Problem Understanding This problem asks us to determine whether a robot moving on an infinite two dimensional plane will remain within some bounded region if it repeats a sequence of instructions forever. The robot starts at coordinate (0, 0) facing north. The input is a string called instructions , where each character represents an action: 'G' means move forward...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 721 - Accounts Merge

LeetCode Problem 721 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Depth-First Search, Breadth-First Search, Union-Find, Sorting Solution Problem Understanding This problem asks us to merge user accounts based on shared email addresses. Each account is represented as a list of strings. The first string is the user's name, and every remaining string is an email address associated with that account. The important detail is that two accounts belong to...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 159 - Longest Substring with At Most Two Distinct Characters

LeetCode Problem 159 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to find the length of the longest contiguous substring in a string s that contains at most two distinct characters. A substring is a continuous sequence of characters inside the original string. The phrase "at most two distinct characters" means the substring may contain one unique character or two unique characters,...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1647 - Minimum Deletions to Make Character Frequencies Unique

LeetCode Problem 1647 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Sorting Solution Problem Understanding The problem asks us to make a string good , which means no two distinct characters in the string have the same frequency. The input is a string s consisting of lowercase English letters, and the output is the minimum number of character deletions required to achieve a good string. The frequency of a character...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 472 - Concatenated Words

LeetCode Problem 472 Difficulty: 🔴 Hard Topics: Array, String, Dynamic Programming, Depth-First Search, Trie, Sorting Solution Problem Understanding The problem asks us to identify all words in a given list that can be formed by concatenating at least two other words from the same list. In other words, each concatenated word must be fully composed of smaller words that exist in the input array. The input is a list of...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1763 - Longest Nice Substring

LeetCode Problem 1763 Difficulty: 🟢 Easy Topics: Hash Table, String, Divide and Conquer, Bit Manipulation, Sliding Window Solution Problem Understanding The problem asks us to find the longest "nice" substring within a given string s . A substring is considered nice if for every letter it contains, both its lowercase and uppercase forms appear. For example, "aAa" is nice because it contains 'a' and 'A' . Conversely, "abA" is not...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 806 - Number of Lines To Write String

LeetCode Problem 806 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding This problem is asking us to determine how to write a string s on multiple lines when each character has a specific pixel width and no line can exceed 100 pixels. We are given an array widths of length 26, where each index corresponds to a lowercase English letter, and its value indicates how many pixels wide that...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 68 - Text Justification

LeetCode Problem 68 Difficulty: 🔴 Hard Topics: Array, String, Simulation Solution LeetCode 68 - Text Justification Problem Understanding This problem asks us to simulate how text is formatted in a text editor when using full justification. We are given an array of words and a target line width called maxWidth . The goal is to arrange these words into multiple lines such that every line contains exactly maxWidth characters. The...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 890 - Find and Replace Pattern

LeetCode Problem 890 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem gives us a list of words and a target pattern string. We need to determine which words follow the exact same structural character relationship as the pattern. A word matches the pattern if there exists a one to one mapping between characters in the pattern and characters in the word. This mapping must be...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1404 - Number of Steps to Reduce a Number in Binary Representation to One

LeetCode Problem 1404 Difficulty: 🟡 Medium Topics: String, Bit Manipulation, Simulation Solution Problem Understanding The problem gives us a binary number represented as a string s . Our task is to determine how many operations are required to reduce this number to exactly 1 . The allowed operations depend on whether the current value is even or odd: If the number is even, divide it by 2 If the number...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 411 - Minimum Unique Word Abbreviation

LeetCode Problem 411 Difficulty: 🔴 Hard Topics: Array, String, Backtracking, Bit Manipulation Solution LeetCode 411 - Minimum Unique Word Abbreviation Problem Understanding This problem asks us to generate the shortest possible abbreviation for a given target word such that the abbreviation cannot also represent any word in the dictionary. An abbreviation replaces one or more non-adjacent substrings with their lengths. For example, the word "apple" can become: "5" because the...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1396 - Design Underground System

LeetCode Problem 1396 Difficulty: 🟡 Medium Topics: Hash Table, String, Design Solution Problem Understanding This problem asks us to design a small tracking system for an underground railway network. The system must support three operations: Recording when a customer checks into a station. Recording when a customer checks out from a station. Calculating the average travel time between two stations. Each customer is identified by a unique integer id ....

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1028 - Recover a Tree From Preorder Traversal

LeetCode Problem 1028 Difficulty: 🔴 Hard Topics: String, Tree, Depth-First Search, Binary Tree Solution Problem Understanding The problem provides a string traversal representing a preorder depth-first traversal of a binary tree, where each node is represented by its value preceded by D dashes, with D being the depth of the node in the tree. The root node has depth 0 and is not preceded by any dash. If a node...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 537 - Complex Number Multiplication

LeetCode Problem 537 Difficulty: 🟡 Medium Topics: Math, String, Simulation Solution Problem Understanding This problem asks us to multiply two complex numbers represented as strings and return the result in the same string format. A complex number is written as: "real+imaginaryi" For example: "1+1i" represents $1 + i$ "1+-1i" represents $1 - i$ The input strings always follow a valid format, meaning we do not need to worry about malformed...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 166 - Fraction to Recurring Decimal

LeetCode Problem 166 Difficulty: 🟡 Medium Topics: Hash Table, Math, String Solution Problem Understanding This problem asks us to convert a fraction, represented by an integer numerator and an integer denominator , into its decimal string representation. If the decimal expansion contains a repeating fractional sequence, we must enclose the repeating part in parentheses. For example, the fraction 1/2 produces "0.5" because it terminates. The fraction 1/3 produces "0.(3)" because...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 516 - Longest Palindromic Subsequence

LeetCode Problem 516 Difficulty: 🟡 Medium Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to determine the length of the longest palindromic subsequence within a given string s . A palindromic subsequence is a sequence of characters that reads the same forwards and backwards and can be obtained by deleting zero or more characters from the original string without changing the order of the remaining characters. For...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 38 - Count and Say

LeetCode Problem 38 Difficulty: 🟡 Medium Topics: String Solution LeetCode 38, Count and Say Problem Understanding The problem asks us to generate the n th term of the "count-and-say" sequence. This sequence is built recursively, where each term is created by describing the previous term. The sequence starts with: 1 To generate the next term, we read the current string from left to right and describe groups of consecutive identical...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 942 - DI String Match

LeetCode Problem 942 Difficulty: 🟢 Easy Topics: Array, Two Pointers, String, Greedy Solution Problem Understanding In this problem, we are given a string s consisting only of the characters 'I' and 'D' . The string has length n , and we must construct a permutation of the integers from 0 to n , inclusive. A permutation means every number in the range appears exactly once. Since the range is [0,...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 984 - String Without AAA or BBB

LeetCode Problem 984 Difficulty: 🟡 Medium Topics: String, Greedy Solution Problem Understanding The problem gives us two integers, a and b , representing how many 'a' characters and 'b' characters we must place into a string. Our task is to construct any valid string of length a + b such that: Exactly a characters are 'a' Exactly b characters are 'b' The substring "aaa" never appears The substring "bbb" never...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 842 - Split Array into Fibonacci Sequence

LeetCode Problem 842 Difficulty: 🟡 Medium Topics: String, Backtracking Solution LeetCode 842 - Split Array into Fibonacci Sequence Problem Understanding The problem gives us a string consisting only of digits, and asks us to split it into a sequence of integers that behaves like a Fibonacci sequence. A Fibonacci-like sequence follows three important rules: Every number must fit inside a signed 32-bit integer, meaning each value must be less than...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 394 - Decode String

LeetCode Problem 394 Difficulty: 🟡 Medium Topics: String, Stack, Recursion Solution LeetCode 394 - Decode String Problem Understanding The problem gives us a string that contains encoded patterns of the form k[encoded_string] . The integer k tells us how many times the substring inside the brackets should be repeated. Our task is to fully decode the string and return the expanded result. For example, the input: 3[a] means the string...

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LeetCode 1332 - Remove Palindromic Subsequences

LeetCode Problem 1332 Difficulty: 🟢 Easy Topics: Two Pointers, String Solution Problem Understanding The problem gives us a string s that contains only the characters 'a' and 'b' . In one operation, we are allowed to remove any subsequence of the string, as long as that subsequence forms a palindrome. A subsequence is different from a substring. A substring must be contiguous, but a subsequence can skip characters while preserving...

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LeetCode 709 - To Lower Case

LeetCode Problem 709 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to convert every uppercase English letter in a string into its lowercase equivalent. Any character that is already lowercase, or is not an alphabetic character at all, should remain unchanged. The input is a string s containing printable ASCII characters. Printable ASCII includes uppercase letters, lowercase letters, digits, punctuation, spaces, and symbols. The task is...

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LeetCode 1152 - Analyze User Website Visit Pattern

LeetCode Problem 1152 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Here is a comprehensive, detailed technical solution guide for LeetCode 1152 - Analyze User Website Visit Pattern following your requested format exactly. Problem Understanding The problem asks us to analyze user website visit sequences to determine the most common pattern of length three. We are given three arrays of equal length: username , timestamp , and website...

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LeetCode 20 - Valid Parentheses

LeetCode Problem 20 Difficulty: 🟢 Easy Topics: String, Stack Solution Problem Understanding The problem gives a string containing only six possible characters: ( , ) , { , } , [ and ] . Each opening bracket must eventually be matched with the correct closing bracket, and the order of matching matters. A string is considered valid only if every opening bracket is closed by the same type of bracket...

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LeetCode 1456 - Maximum Number of Vowels in a Substring of Given Length

LeetCode Problem 1456 Difficulty: 🟡 Medium Topics: String, Sliding Window Solution Problem Understanding This problem asks us to determine the maximum number of vowels that appear in any substring of a given length k within a string s . The input string s consists only of lowercase English letters, and k is guaranteed to be at least 1 and no larger than the length of the string. Vowels are defined...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 926 - Flip String to Monotone Increasing

LeetCode Problem 926 Difficulty: 🟡 Medium Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to transform a binary string into a monotone increasing string using the minimum number of character flips. A binary string is considered monotone increasing if all 0 s appear before all 1 s. In other words, once a 1 appears, no 0 can appear after it. Valid examples include: "000111" "111" "000" ""...

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LeetCode 1507 - Reformat Date

LeetCode Problem 1507 Difficulty: 🟢 Easy Topics: String Solution LeetCode 1507 - Reformat Date Problem Understanding The problem gives a date string in a human readable format such as "20th Oct 2052" and asks us to convert it into the standardized ISO style format YYYY-MM-DD . The input string always contains exactly three parts: A day value with an ordinal suffix such as "1st" , "2nd" , "3rd" , "4th"...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 753 - Cracking the Safe

LeetCode Problem 753 Difficulty: 🔴 Hard Topics: String, Depth-First Search, Graph Theory, Eulerian Circuit Solution Problem Understanding This problem asks us to generate the shortest possible string that guarantees a safe will unlock at some point while typing it. The safe password has exactly n digits, and each digit can be any value from 0 to k - 1 . The safe does not validate the entire entered sequence at...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 640 - Solve the Equation

LeetCode Problem 640 Difficulty: 🟡 Medium Topics: Math, String, Simulation Solution Problem Understanding The problem is asking us to solve a linear equation containing a single variable 'x' and integer coefficients, expressed as a string. The equation may include addition '+' , subtraction '-' , and the equality operator '=' . We need to parse this string, combine like terms, and solve for 'x' . The result should be returned...

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LeetCode 648 - Replace Words

LeetCode Problem 648 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie Solution Problem Understanding The problem asks us to replace words in a sentence using a set of predefined root words . A root is a shorter word that can serve as a prefix for a longer derivative word. Whenever a word in the sentence starts with one of the roots from the dictionary, we replace that word with...

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LeetCode 79 - Word Search

LeetCode Problem 79 Difficulty: 🟡 Medium Topics: Array, String, Backtracking, Depth-First Search, Matrix Solution Problem Understanding In this problem, we are given a two dimensional grid of characters called board and a target string called word . The task is to determine whether the word can be formed by traversing the grid under a strict set of movement rules. A valid word construction must satisfy the following conditions: You may...

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LeetCode 1400 - Construct K Palindrome Strings

LeetCode Problem 1400 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Counting Solution Problem Understanding The problem is asking whether a given string s can be rearranged to form exactly k non-empty palindrome strings using all characters in s . In other words, we need to partition s into k palindromes such that each character of s is used exactly once. A palindrome is a string that reads the same...

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LeetCode 1593 - Split a String Into the Max Number of Unique Substrings

LeetCode Problem 1593 Difficulty: 🟡 Medium Topics: Hash Table, String, Backtracking Solution LeetCode 1593 - Split a String Into the Max Number of Unique Substrings Problem Understanding This problem asks us to split a string into the largest possible number of substrings such that every substring is unique. The substrings must be contiguous pieces of the original string, and when combined in order, they must reconstruct the original string exactly....

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LeetCode 288 - Unique Word Abbreviation

LeetCode Problem 288 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design Solution Problem Understanding The problem asks us to design a data structure that determines whether a word's abbreviation is unique within a given dictionary. An abbreviation is defined using three parts: The first character The number of characters between the first and last character The last character For example: "dog" becomes "d1g" "internationalization" becomes "i18n" "it" remains "it"...

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LeetCode 1604 - Alert Using Same Key-Card Three or More Times in a One Hour Period

LeetCode Problem 1604 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem gives us two parallel arrays, keyName and keyTime . Each index represents a single key-card usage event. For example: keyName[i] = employee name keyTime[i] = time of usage Together, they describe when a specific employee used their key-card during a single day. Our task is to identify every employee who used their key-card...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1271 - Hexspeak

LeetCode Problem 1271 Difficulty: 🟢 Easy Topics: Math, String Solution Problem Understanding The problem asks us to convert a decimal number given as a string into its Hexspeak representation. Hexspeak is derived from the hexadecimal representation of the number, with a simple character substitution: '0' becomes 'O' and '1' becomes 'I' . A valid Hexspeak string can only contain the characters {'A', 'B', 'C', 'D', 'E', 'F', 'I', 'O'} ....

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1374 - Generate a String With Characters That Have Odd Counts

LeetCode Problem 1374 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to generate a string of length n such that each character in the string occurs an odd number of times . The string must consist only of lowercase English letters. The input n is a positive integer ranging from 1 to 500, so the algorithm needs to handle small and moderately large strings efficiently. The...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 949 - Largest Time for Given Digits

LeetCode Problem 949 Difficulty: 🟡 Medium Topics: Array, String, Backtracking, Enumeration Solution Problem Understanding The problem provides an array of exactly four digits. Using each digit exactly once, we must construct the latest possible valid 24-hour time in the format "HH:MM" . A valid 24-hour time has two important constraints: The hour portion, HH , must be between 00 and 23 The minute portion, MM , must be between 00...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 555 - Split Concatenated Strings

LeetCode Problem 555 Difficulty: 🟡 Medium Topics: Array, String, Greedy Solution Problem Understanding The problem gives an array of strings, and we must arrange them into a circular concatenation while preserving the original order of the strings. For every individual string, we are allowed to either keep it as-is or reverse it before joining it into the loop. After constructing this circular string, we may cut the loop at any...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1839 - Longest Substring Of All Vowels in Order

LeetCode Problem 1839 Difficulty: 🟡 Medium Topics: String, Sliding Window Solution Problem Understanding The problem asks us to find the length of the longest substring that is considered "beautiful" according to two strict conditions. First, the substring must contain all five vowels, 'a' , 'e' , 'i' , 'o' , and 'u' , at least once. Second, the vowels inside the substring must appear in non decreasing alphabetical order. This...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1682 - Longest Palindromic Subsequence II

LeetCode Problem 1682 Difficulty: 🟡 Medium Topics: String, Dynamic Programming Solution Problem Understanding The problem is asking us to find the length of the longest good palindromic subsequence in a given string s . A good palindromic subsequence is a string that meets several conditions: it must be a subsequence of s , it must read the same forwards and backwards, it must have an even length , and it...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 972 - Equal Rational Numbers

LeetCode Problem 972 Difficulty: 🔴 Hard Topics: Math, String Solution Problem Understanding The problem asks us to determine whether two strings s and t , each representing a rational number in decimal notation, correspond to the same numerical value. These numbers can be expressed as integers, finite decimals, or decimals with repeating parts denoted by parentheses. For example, 0.1(6) represents 0.1666... repeating indefinitely, while 1. represents the integer 1. The...

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LeetCode 809 - Expressive Words

LeetCode Problem 809 Difficulty: 🟡 Medium Topics: Array, Two Pointers, String Solution Problem Understanding The problem asks us to determine how many words in the words array can be transformed into the target string s using a very specific type of expansion operation. The key concept is that characters in a word can only be expanded if they belong to a group whose final size becomes at least three. A...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 748 - Shortest Completing Word

LeetCode Problem 748 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding The problem gives us a string called licensePlate and an array of candidate words called words . We must find the shortest word that satisfies all the letter requirements contained in licensePlate . The key detail is that only alphabetic characters matter. Digits and spaces should be ignored completely. In addition, uppercase and lowercase letters are...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 212 - Word Search II

LeetCode Problem 212 Difficulty: 🔴 Hard Topics: Array, String, Backtracking, Trie, Matrix Solution Problem Understanding LeetCode 212, LeetCode Word Search II, asks us to find every word from a given dictionary that can be formed inside a 2D character grid. Each word must be built by moving one cell at a time horizontally or vertically. Diagonal movement is not allowed, and a board cell cannot be reused within the same...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 848 - Shifting Letters

LeetCode Problem 848 Difficulty: 🟡 Medium Topics: Array, String, Prefix Sum Solution Problem Understanding This problem asks us to repeatedly apply character shifts to a string of lowercase English letters. We are given two inputs: A string s , consisting only of lowercase English letters. An integer array shifts , where shifts[i] tells us how many times we should shift the first i + 1 characters of the string. A...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 953 - Verifying an Alien Dictionary

LeetCode Problem 953 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding This problem asks us to verify whether a list of words is sorted according to a custom alphabet order, instead of the normal English alphabetical order. In normal lexicographical ordering, characters are compared from left to right using the standard alphabet. Here, the alien language still uses lowercase English letters, but the relative order of those...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1418 - Display Table of Food Orders in a Restaurant

LeetCode Problem 1418 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting, Ordered Set Solution Problem Understanding The problem gives a list of restaurant orders. Each order contains three pieces of information: Customer name Table number Food item An order looks like this: [customerName, tableNumber, foodItem] The goal is to build a display table that summarizes how many times each food item was ordered at every table. The output format...

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LeetCode 1153 - String Transforms Into Another String

LeetCode Problem 1153 Difficulty: 🔴 Hard Topics: Hash Table, String, Graph Theory Solution Problem Understanding The problem gives us two strings, str1 and str2 , which are guaranteed to have the same length. We want to determine whether it is possible to transform str1 into str2 using a sequence of character conversion operations. A single conversion operation works as follows: Choose one character in str1 Replace every occurrence of that...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 916 - Word Subsets

LeetCode Problem 916 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem asks us to determine which strings in words1 satisfy all character requirements imposed by every string in words2 . A string b is considered a subset of another string a if every character in b appears in a at least as many times as it appears in b . This includes repeated characters. For...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1023 - Camelcase Matching

LeetCode Problem 1023 Difficulty: 🟡 Medium Topics: Array, Two Pointers, String, Trie, String Matching Solution Problem Understanding The problem gives us a list of query strings and a target camel case pattern. For each query, we must determine whether the query can be formed by inserting only lowercase English letters into the pattern. The important restriction is that we are only allowed to insert lowercase letters. We are not allowed...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1768 - Merge Strings Alternately

LeetCode Problem 1768 Difficulty: 🟢 Easy Topics: Two Pointers, String Solution Problem Understanding The problem requires merging two input strings word1 and word2 in an alternating fashion, starting with the first character of word1 . This means we take one character from word1 , then one character from word2 , and repeat this process until one or both strings are exhausted. If one string is longer than the other, the...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 126 - Word Ladder II

LeetCode Problem 126 Difficulty: 🔴 Hard Topics: Hash Table, String, Backtracking, Breadth-First Search Solution Problem Understanding This problem asks us to find all shortest transformation sequences from beginWord to endWord , where each transformation changes exactly one character and every intermediate word must exist in wordList . We can think of the problem as navigating through a graph. Each word is a node, and two words are connected if they...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 756 - Pyramid Transition Matrix

LeetCode Problem 756 Difficulty: 🟡 Medium Topics: Hash Table, String, Backtracking, Bit Manipulation Solution Here’s a full, detailed technical solution guide for LeetCode 756 - Pyramid Transition Matrix , following your formatting rules precisely. Problem Understanding The problem is asking us to determine whether a pyramid of blocks can be constructed from a given bottom row of blocks, such that all blocks above obey certain triangular stacking rules. Each block...

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LeetCode 415 - Add Strings

LeetCode Problem 415 Difficulty: 🟢 Easy Topics: Math, String, Simulation Solution Problem Understanding The problem asks us to add two non-negative integers where each integer is given as a string instead of a numeric type. The goal is to return the resulting sum as another string. For example, if the inputs are "11" and "123" , we should return "134" . The important restriction is that we are not allowed...

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LeetCode 606 - Construct String from Binary Tree

LeetCode Problem 606 Difficulty: 🟡 Medium Topics: String, Tree, Depth-First Search, Binary Tree Solution Problem Understanding This problem asks us to convert a binary tree into a string using a very specific preorder traversal format. We must visit nodes in the order root → left subtree → right subtree , and represent the structure of the tree using parentheses. Each node contributes its integer value to the final string. If...

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LeetCode 1156 - Swap For Longest Repeated Character Substring

LeetCode Problem 1156 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to determine the length of the longest substring in a given string text where all characters are the same, and we are allowed to swap exactly two characters in the string. The swap can be between any two characters, not necessarily adjacent, but we are limited to a single swap per...

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LeetCode 1525 - Number of Good Ways to Split a String

LeetCode Problem 1525 Difficulty: 🟡 Medium Topics: Hash Table, String, Dynamic Programming, Bit Manipulation Solution Problem Understanding The problem asks us to count how many ways we can split a string into two non-empty parts such that both parts contain the same number of distinct characters. A split occurs between two adjacent characters. For a string of length n , there are exactly n - 1 possible split positions because...

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LeetCode 67 - Add Binary

LeetCode Problem 67 Difficulty: 🟢 Easy Topics: Math, String, Bit Manipulation, Simulation Solution LeetCode 67, Add Binary Problem Statement You are given two strings, a and b , where each string represents a binary number. A binary number contains only the characters '0' and '1' . The task is to add these two binary numbers together and return the result as another binary string. Unlike normal decimal addition, binary addition...

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LeetCode 227 - Basic Calculator II

LeetCode Problem 227 Difficulty: 🟡 Medium Topics: Math, String, Stack Solution Problem Understanding The problem gives us a string representing a mathematical expression containing non-negative integers, spaces, and the four operators + , - , * , and / . Our task is to evaluate the expression and return the resulting integer value. The important detail is that multiplication and division must follow normal arithmetic precedence rules. This means *...

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LeetCode 1451 - Rearrange Words in a Sentence

LeetCode Problem 1451 Difficulty: 🟡 Medium Topics: String, Sorting Solution Problem Understanding The problem gives us a sentence where words are separated by single spaces, the very first character of the sentence is uppercase, and all remaining characters are lowercase. We must rearrange the words so that they appear in increasing order of word length. If two words have the same length, we must preserve their original relative order from...

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LeetCode 1639 - Number of Ways to Form a Target String Given a Dictionary

LeetCode Problem 1639 Difficulty: 🔴 Hard Topics: Array, String, Dynamic Programming Solution Problem Understanding The problem gives us a list of strings called words , where every string has the same length, and another string called target . We need to count how many different ways we can build target by selecting characters from the columns of words . The important restriction is that we build target from left to...

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LeetCode 1618 - Maximum Font to Fit a Sentence in a Screen

LeetCode Problem 1618 Difficulty: 🟡 Medium Topics: Array, String, Binary Search, Interactive Solution Problem Understanding In this problem, we are given a string text that must be displayed on a single line inside a screen with width w and height h . We are also given a sorted array fonts , where each value represents an available font size that we are allowed to use. The important detail is that...

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LeetCode 1178 - Number of Valid Words for Each Puzzle

LeetCode Problem 1178 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Bit Manipulation, Trie Solution Problem Understanding The problem asks us to determine, for each given puzzle string, how many words from a given list words are valid for that puzzle. A word is valid for a puzzle if it contains the first character of the puzzle and all characters of the word are also present in the puzzle. The...

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LeetCode 1256 - Encode Number

LeetCode Problem 1256 Difficulty: 🟡 Medium Topics: Math, String, Bit Manipulation Solution Sure, let's build a full, detailed technical solution guide for LeetCode 1256 following your requested structure. Problem Understanding The problem asks us to take a non-negative integer num and encode it as a string using a secret encoding rule. The examples indicate that this encoding is not standard binary, decimal, or hexadecimal conversion, but follows a pattern where...

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LeetCode 22 - Generate Parentheses

LeetCode Problem 22 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Backtracking Solution Problem Understanding The problem asks us to generate every possible valid combination of n pairs of parentheses. A valid parentheses string is one where every opening parenthesis ( has a matching closing parenthesis ) , and the parentheses are properly nested. For example, "(()())" is valid because every opening bracket is closed in the correct order. However, strings...

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LeetCode 940 - Distinct Subsequences II

LeetCode Problem 940 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to count how many distinct non-empty subsequences can be formed from a given string s . A subsequence is formed by deleting zero or more characters while keeping the remaining characters in their original order. For example, from "abc" we can form subsequences like "a" , "ac" , and "bc" , but not...

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LeetCode 14 - Longest Common Prefix

LeetCode Problem 14 Difficulty: 🟢 Easy Topics: Array, String, Trie Solution LeetCode 14, Longest Common Prefix Problem Understanding The problem gives an array of strings and asks us to find the longest prefix shared by every string in the array. A prefix is a sequence of characters that appears at the beginning of a string. For example, "fl" is a prefix of "flower" and "flow" because both strings start with...

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LeetCode 344 - Reverse String

LeetCode Problem 344 Difficulty: 🟢 Easy Topics: Two Pointers, String Solution Problem Understanding The problem asks us to reverse a string that is represented as an array of characters. Instead of returning a new reversed string, we must modify the original array directly. This requirement is important because the problem explicitly states that the solution must work in-place and use only O(1) extra memory. The input is an array s...

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LeetCode 816 - Ambiguous Coordinates

LeetCode Problem 816 Difficulty: 🟡 Medium Topics: String, Backtracking, Enumeration Solution Problem Understanding The problem gives us a compressed string representation of a 2D coordinate. Originally, the coordinate looked something like "(1, 3)" or "(2, 0.5)" , but all commas, spaces, and decimal points were removed. Our task is to reconstruct every possible valid original coordinate pair. For example: "(1, 3)" -> "(13)" "(2, 0.5)" -> "(205)" Given the compressed...

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LeetCode 854 - K-Similar Strings

LeetCode Problem 854 Difficulty: 🔴 Hard Topics: Hash Table, String, Breadth-First Search Solution Problem Understanding The problem asks us to determine the minimum number of swaps needed to transform one string into another, where both strings are anagrams of each other. A single operation consists of swapping any two characters in s1 . The goal is to make s1 exactly equal to s2 using as few swaps as possible. Because...

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LeetCode 1554 - Strings Differ by One Character

LeetCode Problem 1554 Difficulty: 🟡 Medium Topics: Hash Table, String, Rolling Hash, Hash Function Solution Problem Understanding This problem gives us an array of unique strings called dict , where every string has the same length. We must determine whether there exists at least one pair of strings that differ by exactly one character at the same position. In other words, we are looking for two strings where: Their lengths...

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LeetCode 521 - Longest Uncommon Subsequence I

LeetCode Problem 521 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to find the length of the longest uncommon subsequence between two strings a and b . To understand the problem clearly, we first need to understand what an uncommon subsequence means. A subsequence is a sequence of characters that can be obtained from a string by deleting zero or more characters without changing the relative...

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LeetCode 87 - Scramble String

LeetCode Problem 87 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution LeetCode 87 - Scramble String Problem Understanding This problem asks us to determine whether one string can be transformed into another using a specific recursive scrambling process. We are given two strings, s1 and s2 , and they are guaranteed to have the same length. The scrambling process works recursively. At every step, we may split a string into...

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LeetCode 1100 - Find K-Length Substrings With No Repeated Characters

LeetCode Problem 1100 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to count how many substrings of length k contain only unique characters. A substring is a contiguous portion of the string. For every possible substring of length k , we must determine whether all characters inside it are distinct. If they are, we count that substring toward the final answer. For...

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LeetCode 385 - Mini Parser

LeetCode Problem 385 Difficulty: 🟡 Medium Topics: String, Stack, Depth-First Search Solution Problem Understanding The problem gives us a string representation of a nested integer structure and asks us to reconstruct the corresponding NestedInteger object hierarchy. A NestedInteger can represent one of two things: A single integer value A list of other NestedInteger objects The input string uses a format similar to JSON arrays. For example: "324" represents a single...

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LeetCode 389 - Find the Difference

LeetCode Problem 389 Difficulty: 🟢 Easy Topics: Hash Table, String, Bit Manipulation, Sorting Solution Problem Understanding This problem gives us two strings, s and t . The string t is created by taking all characters from s , shuffling them into a different order, and then inserting exactly one additional character somewhere in the string. Our task is to identify and return that extra character. In other words, every character...

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LeetCode 639 - Decode Ways II

LeetCode Problem 639 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to determine the number of ways an encoded string can be decoded into letters, where 'A' maps to "1" , 'B' maps to "2" , up to 'Z' mapping to "26" . The input string s contains digits '0'-'9' and potentially the wildcard character '*' , which represents any digit from '1' to...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1625 - Lexicographically Smallest String After Applying Operations

LeetCode Problem 1625 Difficulty: 🟡 Medium Topics: String, Depth-First Search, Breadth-First Search, Enumeration Solution Problem Understanding The problem gives us a numeric string s of even length and two operations that can be performed any number of times in any order. The first operation adds the integer a to every digit located at an odd index. Since digits must remain between 0 and 9 , the addition wraps around using...

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LeetCode 468 - Validate IP Address

LeetCode Problem 468 Difficulty: 🟡 Medium Topics: String Solution Problem Understanding The problem asks us to determine whether a given string represents a valid IPv4 address, a valid IPv6 address, or neither. We are given a single string called queryIP , and we must return one of three exact strings: "IPv4" if the input is a valid IPv4 address "IPv6" if the input is a valid IPv6 address "Neither" if...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1737 - Change Minimum Characters to Satisfy One of Three Conditions

LeetCode Problem 1737 Difficulty: 🟡 Medium Topics: Hash Table, String, Counting, Prefix Sum Solution Problem Understanding The problem gives us two lowercase English strings, a and b . In a single operation, we may change any character in either string into any other lowercase English letter. Our task is to determine the minimum number of character changes required so that at least one of the following conditions becomes true: Every...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 588 - Design In-Memory File System

LeetCode Problem 588 Difficulty: 🔴 Hard Topics: Hash Table, String, Design, Trie, Sorting Solution Problem Understanding The problem asks us to design an in-memory file system that simulates basic file system operations without interacting with the real filesystem. We need to implement a FileSystem class that supports creating directories, adding and reading file content, and listing files or directories in a given path. The input consists of paths, which are...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 399 - Evaluate Division

LeetCode Problem 399 Difficulty: 🟡 Medium Topics: Array, String, Depth-First Search, Breadth-First Search, Union-Find, Graph Theory, Shortest Path Solution Problem Understanding This problem gives us a collection of equations between variables, where each equation represents a division relationship. For example, if we are given: a / b = 2.0 b / c = 3.0 then we can infer additional relationships such as: a / c = 6.0 c / a...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1427 - Perform String Shifts

LeetCode Problem 1427 Difficulty: 🟢 Easy Topics: Array, Math, String Solution Problem Understanding This problem asks us to simulate a sequence of string shift operations on a given string s . Each operation in the shift matrix contains two values: direction 0 means shift left 1 means shift right amount the number of positions to shift A left shift removes characters from the front and appends them to the back....

Practice › LeetCode › LeetCode 08xx ›

LeetCode 893 - Groups of Special-Equivalent Strings

LeetCode Problem 893 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem asks us to group strings based on a special equivalence property. Specifically, two strings are special-equivalent if you can swap characters at even indices among themselves and characters at odd indices among themselves any number of times to make the two strings identical. The input is an array of strings words , all...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1268 - Search Suggestions System

LeetCode Problem 1268 Difficulty: 🟡 Medium Topics: Array, String, Binary Search, Trie, Sorting, Heap (Priority Queue) Solution LeetCode 1268 - Search Suggestions System Problem Understanding This problem asks us to build a product suggestion system similar to what appears in e commerce search bars. We are given two inputs: products , an array of unique product names searchWord , the word the user types character by character After each character...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 405 - Convert a Number to Hexadecimal

LeetCode Problem 405 Difficulty: 🟢 Easy Topics: Math, String, Bit Manipulation Solution Problem Understanding The problem asks us to convert a given 32-bit integer num into its hexadecimal representation as a string. Hexadecimal, or base-16, uses digits 0-9 and letters a-f to represent values 0-15 . For positive numbers, the conversion is straightforward: repeatedly divide the number by 16 and map remainders to the hexadecimal digits. For negative numbers, the...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 282 - Expression Add Operators

LeetCode Problem 282 Difficulty: 🔴 Hard Topics: Math, String, Backtracking Solution Problem Understanding The problem gives us a numeric string num and an integer target . We must insert binary operators, specifically '+' , '-' , and '*' , between the digits of the string so that the resulting mathematical expression evaluates exactly to target . The important detail is that we are not allowed to reorder digits. We may...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 49 - Group Anagrams

LeetCode Problem 49 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem asks us to group together all strings that are anagrams of each other. Two strings are considered anagrams if they contain exactly the same characters with the same frequencies, but possibly in a different order. For example, "eat" , "tea" , and "ate" are all anagrams because each string contains one 'a' ,...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1312 - Minimum Insertion Steps to Make a String Palindrome

LeetCode Problem 1312 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Edit LeetCode 1312 - Minimum Insertion Steps to Make a String Palindrome Problem Understanding The problem gives us a string s and asks for the minimum number of insertion operations required to transform the string into a palindrome. In a single operation, we may insert any lowercase English character at any position in the string. The inserted character does...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1759 - Count Number of Homogenous Substrings

LeetCode Problem 1759 Difficulty: 🟡 Medium Topics: Math, String Solution Problem Understanding The problem asks us to count all substrings in a string where every character inside the substring is identical. These are called homogenous substrings. A substring must be contiguous, meaning the characters must appear next to each other in the original string. For example, in the string "abbcccaa" , the substring "ccc" is valid because all characters are...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 844 - Backspace String Compare

LeetCode Problem 844 Difficulty: 🟢 Easy Topics: Two Pointers, String, Stack, Simulation Solution LeetCode 844 - Backspace String Compare Problem Understanding This problem asks us to determine whether two strings become identical after processing backspace operations. Each string represents characters typed into a text editor, where the '#' character means "delete the previous character", similar to pressing the backspace key on a keyboard. We are given two input strings, s...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 730 - Count Different Palindromic Subsequences

LeetCode Problem 730 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding This problem asks us to count how many distinct non-empty palindromic subsequences exist in a string. A subsequence is formed by deleting characters while preserving the relative order of the remaining characters. Unlike substrings, subsequences do not need to be contiguous. A palindrome reads the same forward and backward. The important detail is that we are counting...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 903 - Valid Permutations for DI Sequence

LeetCode Problem 903 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Prefix Sum Solution Problem Understanding The problem gives us a string s of length n , where each character describes the relationship between two adjacent elements in a permutation. The permutation must contain every integer from 0 to n exactly once, so the permutation length is always n + 1 . Each character in the string imposes a constraint: 'I'...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 249 - Group Shifted Strings

LeetCode Problem 249 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem asks us to group strings that belong to the same cyclic shifting sequence. Two strings belong to the same group if one can be transformed into the other by repeatedly shifting every character forward or backward in the alphabet, with wraparound between 'z' and 'a' . For example, "abc" becomes "bcd" after one right...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1058 - Minimize Rounding Error to Meet Target

LeetCode Problem 1058 Difficulty: 🟡 Medium Topics: Array, Math, String, Greedy, Sorting Solution Problem Understanding The problem asks us to round an array of decimal prices to integers such that the sum of the rounded numbers equals a given target, while minimizing the total rounding error. Each price can be rounded either down ( Floor ) or up ( Ceil ). The rounding error for each price is defined as...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 168 - Excel Sheet Column Title

LeetCode Problem 168 Difficulty: 🟢 Easy Topics: Math, String Solution Problem Understanding The problem asks us to convert a positive integer into the column naming format used by Microsoft Excel. Excel labels columns alphabetically instead of numerically. The sequence begins as: 1 -> A 2 -> B ... 26 -> Z 27 -> AA 28 -> AB The input, columnNumber , represents a 1-based column index. Our goal is to...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 449 - Serialize and Deserialize BST

LeetCode Problem 449 Difficulty: 🟡 Medium Topics: String, Tree, Depth-First Search, Breadth-First Search, Design, Binary Search Tree, Binary Tree Solution Problem Understanding The problem asks us to design two operations for a Binary Search Tree, abbreviated as BST: serialize(root) converts the BST into a string representation. deserialize(data) reconstructs the exact same BST from that string. The important requirement is that the deserialized tree must match the original tree structure and...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 44 - Wildcard Matching

LeetCode Problem 44 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Greedy, Recursion Solution Problem Understanding This problem asks us to determine whether an entire input string s matches a wildcard pattern p . The pattern supports two special wildcard characters: '?' matches exactly one character. '*' matches any sequence of characters, including an empty sequence. The important detail is that the match must cover the entire string. Partial matches are...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 541 - Reverse String II

LeetCode Problem 541 Difficulty: 🟢 Easy Topics: Two Pointers, String Solution Problem Understanding The problem requires us to manipulate a string s in a structured way. Specifically, we need to reverse the first k characters for every consecutive block of 2k characters in the string. If a block has fewer than k characters, we reverse all of them. If a block has between k and 2k characters, we reverse the...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1578 - Minimum Time to Make Rope Colorful

LeetCode Problem 1578 Difficulty: 🟡 Medium Topics: Array, String, Dynamic Programming, Greedy Solution Problem Understanding The problem requires transforming a rope of balloons into a "colorful" rope, which means no two consecutive balloons share the same color. The input provides a string colors representing the colors of the balloons in order and an array neededTime representing the time in seconds it takes to remove each corresponding balloon. The task is...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 17 - Letter Combinations of a Phone Number

LeetCode Problem 17 Difficulty: 🟡 Medium Topics: Hash Table, String, Backtracking Solution Problem Understanding The problem gives a string of digits where each digit is between 2 and 9 . Each digit corresponds to a set of letters on a traditional phone keypad: 2 -> "abc" 3 -> "def" 4 -> "ghi" 5 -> "jkl" 6 -> "mno" 7 -> "pqrs" 8 -> "tuv" 9 -> "wxyz" We must generate...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1750 - Minimum Length of String After Deleting Similar Ends

LeetCode Problem 1750 Difficulty: 🟡 Medium Topics: Two Pointers, String Solution Problem Understanding The problem gives us a string s containing only the characters 'a' , 'b' , and 'c' . We may repeatedly perform a deletion operation on the string. In each operation, we must remove: A non-empty prefix where every character is the same A non-empty suffix where every character is the same Both groups must contain the...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 331 - Verify Preorder Serialization of a Binary Tree

LeetCode Problem 331 Difficulty: 🟡 Medium Topics: String, Stack, Tree, Binary Tree Solution Problem Understanding The problem gives us a string that represents the preorder serialization of a binary tree. Each value is separated by commas. A normal integer represents a real tree node, while the character '#' represents a null pointer. In preorder traversal, we always visit nodes in this order: Current node Left subtree Right subtree For example,...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 132 - Palindrome Partitioning II

LeetCode Problem 132 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to split a string into substrings such that every substring is a palindrome. Among all valid palindrome partitions, we must return the minimum number of cuts required. A cut divides the string into two parts. For example, if the string is "aab" , we can partition it as: "a" | "a" | "b"...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 770 - Basic Calculator IV

LeetCode Problem 770 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Stack, Recursion Solution Problem Understanding The problem requires parsing and evaluating a mathematical expression that includes integers, variables, addition, subtraction, multiplication, and parentheses, while also applying a substitution map for certain variables. Essentially, we are building a symbolic algebra engine that simplifies expressions according to standard arithmetic rules. The input consists of a string expression that may contain integers,...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 291 - Word Pattern II

LeetCode Problem 291 Difficulty: 🟡 Medium Topics: Hash Table, String, Backtracking Solution Problem Understanding The problem asks us to determine whether a given string s can be formed by substituting each character in a pattern with a non-empty string , under a bijective mapping . Bijective means that every character maps to exactly one unique substring and no two characters map to the same substring. Essentially, each character in the...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 91 - Decode Ways

LeetCode Problem 91 Difficulty: 🟡 Medium Topics: String, Dynamic Programming Solution Problem Understanding This problem asks us to determine how many different valid ways a numeric string can be decoded into letters using the mapping: "1" → 'A' "2" → 'B' ... "26" → 'Z' The input is a string s containing only digits. Each digit or pair of digits may represent a letter if it forms a valid number...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 318 - Maximum Product of Word Lengths

LeetCode Problem 318 Difficulty: 🟡 Medium Topics: Array, String, Bit Manipulation Solution LeetCode 318 - Maximum Product of Word Lengths Problem Understanding The problem gives us an array of lowercase English words. We need to find two different words such that they do not share any common letters, then return the maximum possible product of their lengths. More formally, for every pair of indices (i, j) where i != j...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 720 - Longest Word in Dictionary

LeetCode Problem 720 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie, Sorting Solution LeetCode 720, Longest Word in Dictionary Problem Understanding This problem gives us an array of lowercase English words and asks us to find the longest word that can be constructed one character at a time using other words from the same dictionary. More specifically, suppose we have a candidate word like "world" . For this word...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 796 - Rotate String

LeetCode Problem 796 Difficulty: 🟢 Easy Topics: String, String Matching Solution Problem Understanding The problem asks us to determine whether one string can be transformed into another string using repeated left rotations. A single shift operation removes the first character of the string and appends it to the end. For example: "abcde" becomes "bcdea" after one shift "bcdea" becomes "cdeab" after another shift We are given two strings, s and...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 424 - Longest Repeating Character Replacement

LeetCode Problem 424 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem gives us a string s consisting only of uppercase English letters and an integer k . We are allowed to perform at most k replacement operations. In one operation, we can change any character in the string into any other uppercase English character. The goal is to find the length of the longest...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1297 - Maximum Number of Occurrences of a Substring

LeetCode Problem 1297 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Please provide the missing problem number or title you want the guide for. Your prompt contains the formatting/template and problem statement for LeetCode 1297 - Maximum Number of Occurrences of a Substring , so I assume you want the complete technical solution guide for that problem. If yes, I will provide the full guide with all required...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 428 - Serialize and Deserialize N-ary Tree

LeetCode Problem 428 Difficulty: 🔴 Hard Topics: String, Tree, Depth-First Search, Breadth-First Search Solution Problem Understanding LeetCode 428, LeetCode Serialize and Deserialize N-ary Tree, asks us to design a reversible encoding system for an N-ary tree. The problem is not asking for a specific serialization format. Instead, it asks us to guarantee that after serializing a tree into a string and then deserializing that string, we reconstruct the exact same...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1096 - Brace Expansion II

LeetCode Problem 1096 Difficulty: 🔴 Hard Topics: Hash Table, String, Backtracking, Stack, Breadth-First Search, Sorting Solution Problem Understanding The problem asks us to interpret a string expression representing a set of words generated according to a specific grammar and return all distinct words sorted in lexicographical order. The expression can contain lowercase letters, curly braces {} , and commas , . The rules of the grammar are: A single lowercase...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 821 - Shortest Distance to a Character

LeetCode Problem 821 Difficulty: 🟢 Easy Topics: Array, Two Pointers, String Solution Problem Understanding The problem gives us a string s and a target character c . For every index in the string, we must compute the distance to the nearest occurrence of c . The distance between two indices is defined as: $$|i - j|$$ where i is the current index and j is the index of some occurrence...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 395 - Longest Substring with At Least K Repeating Characters

LeetCode Problem 395 Difficulty: 🟡 Medium Topics: Hash Table, String, Divide and Conquer, Sliding Window Solution Problem Understanding The problem asks us to find the length of the longest substring in which every character appears at least k times. A substring is a contiguous sequence of characters inside the original string. For any candidate substring, we must verify that every distinct character inside that substring has frequency greater than or...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 58 - Length of Last Word

LeetCode Problem 58 Difficulty: 🟢 Easy Topics: String Solution LeetCode 58, Length of Last Word Problem Understanding The problem gives us a string s that contains English letters and spaces. Inside this string, words are separated by spaces, and there may also be extra spaces at the beginning or end of the string. Our task is to return the length of the last word in the string. A word is...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1138 - Alphabet Board Path

LeetCode Problem 1138 Difficulty: 🟡 Medium Topics: Hash Table, String Solution Problem Understanding The problem gives us a special alphabet board and asks us to generate the shortest sequence of moves needed to spell a target string. The board contains lowercase English letters arranged in rows: abcde fghij klmno pqrst uvwxy z We always begin at position (0, 0) , which corresponds to the character 'a' . From any valid...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 293 - Flip Game

LeetCode Problem 293 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding This problem is asking us to simulate a single move in a simple two-player game. The input is a string currentState consisting only of '+' and '-' characters. Each '+' represents a spot that can be flipped, and the rules allow a player to flip two consecutive '+' characters into '--' . The task is to generate all possible...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1092 - Shortest Common Supersequence

LeetCode Problem 1092 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Shortest Common Supersequence Problem Understanding The problem asks us to construct the shortest possible string that contains both str1 and str2 as subsequences. A subsequence does not require characters to appear contiguously. Instead, the characters only need to appear in the same relative order. For example, "abc" is a subsequence of "axbxc" because we can delete the extra characters....

Practice › LeetCode › LeetCode 02xx ›

LeetCode 245 - Shortest Word Distance III

LeetCode Problem 245 Difficulty: 🟡 Medium Topics: Array, String Solution Problem Understanding The problem gives us an array of strings, wordsDict , and two target words, word1 and word2 . We must find the minimum distance between any occurrence of these two words in the array. The distance between two words is defined as the absolute difference between their indices in the array. For example, if "coding" appears at index...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 616 - Add Bold Tag in String

LeetCode Problem 616 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie, String Matching Solution Problem Understanding The problem asks us to insert HTML-style bold tags into a string whenever a substring matches any word from a given dictionary of words. We are given: A string s An array words , where each element is a pattern we want to highlight For every occurrence of every word inside s ,...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 925 - Long Pressed Name

LeetCode Problem 925 Difficulty: 🟢 Easy Topics: Two Pointers, String Solution Problem Understanding In this problem, we are given two strings, name and typed . The string name represents the intended sequence of characters your friend wanted to type. The string typed represents the actual characters that appeared on the screen. The complication is that some keys may have been long pressed. A long press means that when a character...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 524 - Longest Word in Dictionary through Deleting

LeetCode Problem 524 Difficulty: 🟡 Medium Topics: Array, Two Pointers, String, Sorting Solution Problem Understanding The problem gives us a source string s and a list of candidate words called dictionary . We must determine which dictionary word can be formed by deleting characters from s without changing the relative order of the remaining characters. This means we are looking for a subsequence relationship. A word is valid if we...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1657 - Determine if Two Strings Are Close

LeetCode Problem 1657 Difficulty: 🟡 Medium Topics: Hash Table, String, Sorting, Counting Solution Problem Understanding The problem asks whether two strings, word1 and word2 , are "close" according to two allowed operations. Operation 1 allows swapping any two existing characters in the string, which means the relative order of characters is flexible. Operation 2 allows transforming all occurrences of one character into another existing character, and vice versa, which effectively...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 345 - Reverse Vowels of a String

LeetCode Problem 345 Difficulty: 🟢 Easy Topics: Two Pointers, String Solution Problem Understanding The problem asks us to reverse only the vowels in a string while leaving all non-vowel characters in their original positions. A vowel is defined as one of the following characters: a e i o u The problem also states that vowels may appear in both lowercase and uppercase forms. This means characters like A , E...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1044 - Longest Duplicate Substring

LeetCode Problem 1044 Difficulty: 🔴 Hard Topics: String, Binary Search, Sliding Window, Rolling Hash, Suffix Array, Hash Function Solution Here is the complete, detailed technical solution guide for LeetCode 1044 - Longest Duplicate Substring, following your formatting rules exactly. Problem Understanding The problem asks us to find the longest duplicated substring in a given string s . A duplicated substring is a contiguous sequence of characters that appears at least...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1189 - Maximum Number of Balloons

LeetCode Problem 1189 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem asks us to determine the maximum number of times the word "balloon" can be formed using the characters from a given string text . Each character in text can be used at most once per occurrence of the word. The input is a string consisting of only lowercase English letters, with length up to...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1258 - Synonymous Sentences

LeetCode Problem 1258 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Backtracking, Sort, Union-Find Solution Problem Understanding This problem asks us to generate all possible synonymous variations of a given sentence based on a list of equivalent word pairs. Each synonym pair indicates that the two words can replace each other interchangeably. The output must include all possible sentences created by substituting words with their synonyms and must be sorted...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1540 - Can Convert String in K Moves

LeetCode Problem 1540 Difficulty: 🟡 Medium Topics: Hash Table, String Solution Problem Understanding The problem is asking whether it is possible to convert one string s into another string t using at most k moves, following specific shifting rules. Each move allows you to choose a character in s that has not been moved before and shift it forward in the alphabet exactly i times, where i is the move...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 966 - Vowel Spellchecker

LeetCode Problem 966 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem asks us to build a spellchecker with three levels of matching priority. For every query word, we must search the wordlist and return the best matching word according to a strict precedence order. The first and highest priority rule is an exact match. If the query exactly matches a word in the wordlist ,...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1324 - Print Words Vertically

LeetCode Problem 1324 Difficulty: 🟡 Medium Topics: Array, String, Simulation Solution Problem Understanding The problem requires us to take a string s containing multiple words separated by single spaces and return a list of strings representing the words written vertically. In other words, each column of the output corresponds to characters from the original words at the same position. If a word is shorter than the longest word, missing characters...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1717 - Maximum Score From Removing Substrings

LeetCode Problem 1717 Difficulty: 🟡 Medium Topics: String, Stack, Greedy Solution Problem Understanding The problem gives us a string s and two scoring rules: Removing the substring "ab" earns x points. Removing the substring "ba" earns y points. We may perform these removals as many times as possible, in any order we choose. Every time we remove a substring, the remaining characters close together, which may create new removable substrings....

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1694 - Reformat Phone Number

LeetCode Problem 1694 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem requires reformatting a phone number string in a specific structured way. The input is a string number containing digits, spaces, and dashes. Our task is to first remove all non-digit characters, then partition the digits into blocks of length 3, with special handling for the last few digits: if exactly 4 digits remain, split them into two...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 269 - Alien Dictionary

LeetCode Problem 269 Difficulty: 🔴 Hard Topics: Array, String, Depth-First Search, Breadth-First Search, Graph Theory, Topological Sort Solution LeetCode 269, Alien Dictionary Problem Understanding This problem asks us to reconstruct the ordering of characters in an unknown alien language. We are given a list of words that are already sorted according to the alien language's lexicographical order. The alphabet itself still uses lowercase English letters, but the order between letters...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1455 - Check If a Word Occurs As a Prefix of Any Word in a Sentence

LeetCode Problem 1455 Difficulty: 🟢 Easy Topics: Two Pointers, String, String Matching Solution Problem Understanding The problem asks us to determine whether a given searchWord occurs as a prefix of any word within a sentence. A sentence is defined as a string of lowercase English letters separated by single spaces, and a prefix is any contiguous leading substring of a word. We are asked to return the 1-indexed position of...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1813 - Sentence Similarity III

LeetCode Problem 1813 Difficulty: 🟡 Medium Topics: Array, Two Pointers, String Solution Problem Understanding The problem is asking us to determine whether two sentences can be made identical by inserting a contiguous sequence of words (possibly empty) into one of them. Each sentence is a string of words separated by single spaces. Importantly, the insertion must occur between words, not by altering any existing word. The input consists of two...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1698 - Number of Distinct Substrings in a String

LeetCode Problem 1698 Difficulty: 🟡 Medium Topics: String, Trie, Rolling Hash, Suffix Array, Hash Function Solution Problem Understanding The problem asks for the total number of distinct substrings of a given string s . A substring is any contiguous sequence of characters from the string, including single-character substrings and the string itself. The output is an integer representing how many unique substrings can be formed from s . For example,...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 340 - Longest Substring with At Most K Distinct Characters

LeetCode Problem 340 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Problem Understanding The problem asks us to find the length of the longest contiguous substring in a given string s such that the substring contains at most k distinct characters. A substring is a continuous portion of the string. The important condition is not the total number of characters, but the number of unique character types inside...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 767 - Reorganize String

LeetCode Problem 767 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy, Sorting, Heap (Priority Queue), Counting Solution Problem Understanding The problem asks us to rearrange the characters of a given string s such that no two adjacent characters are the same. The input is a string of lowercase English letters with a length between 1 and 500. The output should be either a valid rearrangement that satisfies the adjacency constraint...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1542 - Find Longest Awesome Substring

LeetCode Problem 1542 Difficulty: 🔴 Hard Topics: Hash Table, String, Bit Manipulation Solution Problem Understanding The problem asks us to find the length of the longest awesome substring in a given string s consisting of digits. A substring is considered awesome if it can be rearranged (via any number of swaps) to form a palindrome. In other words, the characters in the substring can be permuted so that it reads...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 833 - Find And Replace in String

LeetCode Problem 833 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem gives us a string s and three parallel arrays: indices[i] tells us where a replacement might happen sources[i] is the substring we expect to find at that index targets[i] is the string we should replace it with if the match is valid For each operation, we must check whether sources[i] actually appears in...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 955 - Delete Columns to Make Sorted II

LeetCode Problem 955 Difficulty: 🟡 Medium Topics: Array, String, Greedy Solution Problem Understanding This problem asks us to remove the minimum number of columns from a list of equal length strings so that the resulting array of strings becomes lexicographically sorted. The important detail is that when we delete a column, we delete that character position from every string simultaneously. We cannot selectively delete characters from individual strings. Lexicographic order...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1181 - Before and After Puzzle

LeetCode Problem 1181 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem gives us an array of phrases, where each phrase is a string made of lowercase English letters and spaces. Every phrase is well-formed, meaning there are no leading or trailing spaces and no consecutive spaces. We must generate all possible "Before and After" puzzles by combining two different phrases. A valid merge happens...

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LeetCode 1576 - Replace All ?'s to Avoid Consecutive Repeating Characters

LeetCode Problem 1576 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding This problem asks us to take a string s consisting of lowercase English letters and the character '?' and replace every '?' with a lowercase letter so that no two consecutive characters in the resulting string are the same . Importantly, we are not allowed to change any existing letters in the string. The input string can be of...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1410 - HTML Entity Parser

LeetCode Problem 1410 Difficulty: 🟡 Medium Topics: Hash Table, String Solution Here is a complete, detailed technical guide for LeetCode 1410 following your formatting and style requirements. Problem Understanding The problem asks us to implement an HTML entity parser . We are given a string text which may contain HTML entities such as &quot; for " or &amp; for & . The goal is to replace all valid entities in...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 838 - Push Dominoes

LeetCode Problem 838 Difficulty: 🟡 Medium Topics: Two Pointers, String, Dynamic Programming Solution Problem Understanding This problem models a chain reaction of falling dominoes. We are given a string where each character represents the initial state of a domino in a row. A domino can be in one of three states: 'L' means the domino has been pushed to the left. 'R' means the domino has been pushed to the...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1653 - Minimum Deletions to Make String Balanced

LeetCode Problem 1653 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Stack Solution Problem Understanding The problem asks us to make a string containing only characters 'a' and 'b' balanced by deleting the minimum number of characters. A string is considered balanced if there are no occurrences where a 'b' comes before an 'a' in the string. In other words, all 'a' characters must appear before any 'b' characters, or equivalently,...

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LeetCode 439 - Ternary Expression Parser

LeetCode Problem 439 Difficulty: 🟡 Medium Topics: String, Stack, Recursion Solution Problem Understanding The problem gives us a string representing a nested ternary expression and asks us to evaluate it. A ternary expression follows the familiar format: condition ? value_if_true : value_if_false The condition is always either 'T' or 'F' . If the condition is 'T' , the expression evaluates to the value before the ':' . If the condition...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 583 - Delete Operation for Two Strings

LeetCode Problem 583 Difficulty: 🟡 Medium Topics: String, Dynamic Programming Solution Problem Understanding The problem asks us to determine the minimum number of deletion steps required to make two strings identical. We are given two input strings, word1 and word2 , and in each step, we can delete exactly one character from either string. The goal is to transform both strings so that their final forms are identical using as...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 899 - Orderly Queue

LeetCode Problem 899 Difficulty: 🔴 Hard Topics: Math, String, Sorting Solution LeetCode 899 - Orderly Queue Problem Understanding This problem gives us a string s and an integer k . We are allowed to repeatedly perform one operation: Choose one of the first k characters of the string. Remove that character from its current position. Append it to the end of the string. Our goal is to determine the lexicographically...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 917 - Reverse Only Letters

LeetCode Problem 917 Difficulty: 🟢 Easy Topics: Two Pointers, String Solution Problem Understanding The problem asks us to reverse only the English letters in a string while keeping all non-letter characters fixed in their original positions. In other words, letters move, but symbols, digits, and punctuation marks stay exactly where they started. For example, consider the string "ab-cd" . The letters are "a" , "b" , "c" , and "d"...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 736 - Parse Lisp Expression

LeetCode Problem 736 Difficulty: 🔴 Hard Topics: Hash Table, String, Stack, Recursion Solution Problem Understanding The problem asks us to evaluate a Lisp-like expression represented as a string. The expression can contain integers, variables, and three special operations: let , add , and mult . An expression evaluates to a single integer value. The challenge is not arithmetic itself, but correctly handling nested scopes and variable shadowing. A let expression...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 691 - Stickers to Spell Word

LeetCode Problem 691 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Dynamic Programming, Backtracking, Bit Manipulation, Memoization, Bitmask Solution Let's dive deep into a detailed technical solution guide for LeetCode 691, following your formatting rules exactly. Problem Understanding The problem asks us to construct a target string using letters cut from an unlimited supply of given stickers, where each sticker is a lowercase word. You can reuse stickers multiple times...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 12 - Integer to Roman

LeetCode Problem 12 Difficulty: 🟡 Medium Topics: Hash Table, Math, String Solution Problem Understanding The problem asks us to convert a positive integer into its Roman numeral representation. Roman numerals use combinations of specific symbols to represent values, and the rules for combining those symbols are strict. Unlike a positional number system such as decimal, Roman numerals are constructed greedily from the largest possible symbol values down to the smallest....

Practice › LeetCode › LeetCode 07xx ›

LeetCode 752 - Open the Lock

LeetCode Problem 752 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Breadth-First Search Solution Problem Understanding The problem presents a lock with four wheels, each wheel labeled from '0' to '9' , which can rotate forwards or backwards. The lock starts at '0000' , and we are given a list of "deadends," which are lock states that will cause the lock to freeze if reached. The goal is to determine...

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LeetCode 43 - Multiply Strings

LeetCode Problem 43 Difficulty: 🟡 Medium Topics: Math, String, Simulation Solution Problem Understanding The problem asks us to multiply two non-negative integers where each integer is provided as a string instead of a numeric type. The result must also be returned as a string. The important restriction is that we cannot directly convert the strings into integers or use any built-in big integer library. In other words, we must manually...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 466 - Count The Repetitions

LeetCode Problem 466 Difficulty: 🔴 Hard Topics: Two Pointers, String, Dynamic Programming Solution Problem Understanding The problem asks us to find the maximum number of times a string str2 can be obtained from another string str1 when both are repeated multiple times. Specifically, str1 is the string s1 repeated n1 times, and str2 is the string s2 repeated n2 times. To "obtain" str2 from str1 means that we can remove...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 856 - Score of Parentheses

LeetCode Problem 856 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem is asking us to compute a score for a string of balanced parentheses according to a set of rules. A balanced parentheses string is one in which every opening parenthesis '(' has a corresponding closing parenthesis ')' and the pairs are properly nested. The rules for scoring are as follows: A single pair "()" scores 1...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 771 - Jewels and Stones

LeetCode Problem 771 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding In this problem, we are given two strings, jewels and stones . The string jewels represents all stone types that are considered jewels. Each character is a unique jewel type. For example, if jewels = "aA" , then both lowercase 'a' and uppercase 'A' are jewel types. The string stones represents all stones we currently own. Every...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1771 - Maximize Palindrome Length From Subsequences

LeetCode Problem 1771 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem gives us two strings, word1 and word2 . We must choose a non-empty subsequence from each string, concatenate them together, and form a palindrome. Our goal is to maximize the length of that palindrome. A subsequence does not require characters to be contiguous. We are allowed to skip characters as long as the remaining characters...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 567 - Permutation in String

LeetCode Problem 567 Difficulty: 🟡 Medium Topics: Hash Table, Two Pointers, String, Sliding Window Solution Problem Understanding This problem asks us to determine whether any permutation of s1 appears as a contiguous substring inside s2 . A permutation means the characters are rearranged, but the frequency of each character remains the same. For example, the permutations of "ab" are "ab" and "ba" . If either of these appears somewhere inside...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1614 - Maximum Nesting Depth of the Parentheses

LeetCode Problem 1614 Difficulty: 🟢 Easy Topics: String, Stack Solution LeetCode 1614, Maximum Nesting Depth of the Parentheses Problem Understanding This problem asks us to compute the maximum nesting depth of parentheses in a valid parentheses string. A valid parentheses string, often abbreviated as VPS, is a string where every opening parenthesis '(' has a matching closing parenthesis ')' , and the parentheses are properly balanced. The string may also...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 839 - Similar String Groups

LeetCode Problem 839 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Depth-First Search, Breadth-First Search, Union-Find Solution Problem Understanding The problem asks us to determine the number of similarity groups within a list of strings. Two strings are defined as similar if they are either identical or if we can swap exactly two letters in one string to make it equal to the other. For example, "tars" and "rats" are...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 777 - Swap Adjacent in LR String

LeetCode Problem 777 Difficulty: 🟡 Medium Topics: Two Pointers, String Solution Problem Understanding The problem gives us two strings, start and result , both consisting only of the characters 'L' , 'R' , and 'X' . We are allowed to transform the start string using only two kinds of moves: Replace "XL" with "LX" Replace "RX" with "XR" The goal is to determine whether it is possible to transform start...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1016 - Binary String With Substrings Representing 1 To N

LeetCode Problem 1016 Difficulty: 🟡 Medium Topics: Hash Table, String, Bit Manipulation, Sliding Window Solution Problem Understanding The problem asks us to determine whether a given binary string s contains all binary representations of integers from 1 to n as substrings . In other words, for every integer i such that 1 <= i <= n , if you convert i to its binary form, that binary string must appear...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1529 - Minimum Suffix Flips

LeetCode Problem 1529 Difficulty: 🟡 Medium Topics: String, Greedy Solution Problem Understanding This problem is asking us to transform an initial binary string s (all zeros) into a target binary string target using a minimum number of suffix flip operations . A suffix flip operation involves picking an index i and flipping every bit from index i to the end of the string. Flipping a bit means turning '0' to...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 451 - Sort Characters By Frequency

LeetCode Problem 451 Difficulty: 🟡 Medium Topics: Hash Table, String, Sorting, Heap (Priority Queue), Bucket Sort, Counting Solution Problem Understanding The problem asks us to rearrange the characters of a string so that characters with higher frequency appear before characters with lower frequency. The output string must group identical characters together, and those groups must appear in descending order of occurrence count. For example, if the input is "tree" ,...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1624 - Largest Substring Between Two Equal Characters

LeetCode Problem 1624 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem asks us to find the length of the longest substring that lies between two identical characters in a given string s . The substring does not include the two identical characters themselves. If no character occurs at least twice, we must return -1 . In other words, we are looking for pairs of identical characters...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1392 - Longest Happy Prefix

LeetCode Problem 1392 Difficulty: 🔴 Hard Topics: String, Rolling Hash, String Matching, Hash Function Solution Problem Understanding The problem asks us to find the longest substring that is both a prefix and a suffix of the given string s , while also ensuring that the substring is not equal to the entire string itself. A prefix is a substring that starts at index 0 , while a suffix is a...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1415 - The k-th Lexicographical String of All Happy Strings of Length n

LeetCode Problem 1415 Difficulty: 🟡 Medium Topics: String, Backtracking Solution LeetCode 1415, The k-th Lexicographical String of All Happy Strings of Length n Problem Understanding The problem asks us to generate strings of length n using only the characters 'a' , 'b' , and 'c' , with one important restriction: no two adjacent characters can be the same. These valid strings are called happy strings. After generating all valid happy...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1592 - Rearrange Spaces Between Words

LeetCode Problem 1592 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem requires rearranging spaces in a given string text so that all words are separated by the maximum possible equal number of spaces , and any leftover spaces are placed at the end of the string. The input text consists of lowercase English letters and spaces, and it is guaranteed to contain at least one word , which...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1754 - Largest Merge Of Two Strings

LeetCode Problem 1754 Difficulty: 🟡 Medium Topics: Two Pointers, String, Greedy Solution Problem Understanding This problem asks us to build the lexicographically largest possible string by repeatedly choosing characters from the front of two given strings. At every step, we may take the first character from either word1 or word2 , append it to the result, and remove it from its original string. The important detail is that we are...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 763 - Partition Labels

LeetCode Problem 763 Difficulty: 🟡 Medium Topics: Hash Table, Two Pointers, String, Greedy Solution Problem Understanding The problem is asking us to partition a string s into as many contiguous parts as possible such that no letter appears in more than one part. In other words, each character in the string should be confined to a single segment. After partitioning, concatenating all the segments in order should reproduce the original...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 680 - Valid Palindrome II

LeetCode Problem 680 Difficulty: 🟢 Easy Topics: Two Pointers, String, Greedy Solution Problem Understanding The problem asks whether a given string s can become a palindrome after deleting at most one character . A palindrome is a string that reads the same forwards and backwards. For example, "aba" and "racecar" are palindromes because their characters match symmetrically from both ends. The input consists of a single string s made only...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 224 - Basic Calculator

LeetCode Problem 224 Difficulty: 🔴 Hard Topics: Math, String, Stack, Recursion Solution Problem Understanding The problem gives us a string representing a mathematical expression containing integers, addition, subtraction, parentheses, and spaces. Our task is to evaluate the expression and return the final integer result. The expression is guaranteed to be valid. That guarantee is extremely important because it simplifies parsing. We do not need to worry about malformed parentheses, invalid...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2011 - Final Value of Variable After Performing Operations

LeetCode Problem 2011 Difficulty: 🟢 Easy Topics: Array, String, Simulation Solution Problem Understanding This problem asks us to simulate a simple programming language with a single integer variable X that starts at 0 . We are given a list of string operations, each of which either increments ( ++X or X++ ) or decrements ( --X or X-- ) the value of X by 1 . The goal is to...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 677 - Map Sum Pairs

LeetCode Problem 677 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Trie Solution Problem Understanding The problem asks us to design a custom data structure that behaves like a map from strings to integers, while also supporting efficient prefix-based sum queries. There are two operations: insert(key, val) This operation stores a string key with an integer value. If the key already exists, its old value must be replaced with the...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 392 - Is Subsequence

LeetCode Problem 392 Difficulty: 🟢 Easy Topics: Two Pointers, String, Dynamic Programming Solution Problem Understanding The problem asks whether string s can be formed from string t by deleting some characters from t without changing the order of the remaining characters. A subsequence does not require characters to be adjacent. The only requirement is that the relative ordering stays the same. For example, "abc" is a subsequence of "ahbgdc" because...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2224 - Minimum Number of Operations to Convert Time

LeetCode Problem 2224 Difficulty: 🟢 Easy Topics: String, Greedy Solution Problem Understanding The problem asks us to calculate the minimum number of operations to convert one 24-hour time string, current , into another, correct . Both inputs are formatted as "HH:MM" , where HH is hours from 00 to 23 and MM is minutes from 00 to 59 . Each operation allows us to increase the current time by 1,...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1108 - Defanging an IP Address

LeetCode Problem 1108 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to transform a valid IPv4 address into its "defanged" version. A defanged IP address is created by replacing every period character "." with the string "[.]" . An IPv4 address consists of four numeric segments separated by periods. For example, "1.1.1.1" contains four numbers separated by three periods. The required transformation changes each separator so...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 32 - Longest Valid Parentheses

LeetCode Problem 32 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Stack Solution Problem Understanding The problem asks us to find the length of the longest contiguous substring that forms a valid parentheses sequence. A valid parentheses sequence is one where every opening parenthesis '(' has a corresponding closing parenthesis ')' , and the pairs are correctly nested. For example, the string "()()" is valid because each opening parenthesis is matched...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 791 - Custom Sort String

LeetCode Problem 791 Difficulty: 🟡 Medium Topics: Hash Table, String, Sorting Solution LeetCode 791, Custom Sort String Problem Understanding The problem gives us two strings, order and s . The string order defines a custom character ordering. Unlike normal alphabetical ordering, the characters in order specify exactly how characters should be prioritized relative to one another. Every character in order is unique, which means there are no conflicts or duplicate...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 631 - Design Excel Sum Formula

LeetCode Problem 631 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Graph Theory, Design, Topological Sort, Matrix Solution Problem Understanding The problem asks us to design a miniature spreadsheet system similar to Microsoft Excel. The spreadsheet contains cells arranged in rows and columns, where rows are numbered starting from 1 and columns are labeled using uppercase letters such as A , B , C , and so on. Each cell...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1513 - Number of Substrings With Only 1s

LeetCode Problem 1513 Difficulty: 🟡 Medium Topics: Math, String Solution Problem Understanding The problem gives a binary string s , consisting only of the characters '0' and '1' . We must count how many substrings contain only '1' characters. A substring is a contiguous section of the string. For example, in the string "111" , the valid substrings made entirely of '1' are: "1" at index 0 "1" at index...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1446 - Consecutive Characters

LeetCode Problem 1446 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to compute the power of a string. The power is defined as the length of the longest contiguous substring that contains only one distinct character. In simpler terms, we need to find the longest streak of the same character appearing consecutively in the string. We are not looking for the most frequent character overall, only...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 165 - Compare Version Numbers

LeetCode Problem 165 Difficulty: 🟡 Medium Topics: Two Pointers, String Solution Problem Understanding This problem asks us to compare two software version numbers represented as strings. Each version consists of one or more numeric revisions separated by dots ( . ). For example, "1.2.10" contains three revisions: 1 , 2 , and 10 . The important detail is that revisions are compared as integers, not strings. This means leading zeros...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 522 - Longest Uncommon Subsequence II

LeetCode Problem 522 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, String, Sorting Solution Problem Understanding The problem asks us to find the length of the longest string in the array that is not a subsequence of any other string in the same array. A subsequence is formed by deleting zero or more characters from a string without changing the order of the remaining characters. For example, "abc" is...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 247 - Strobogrammatic Number II

LeetCode Problem 247 Difficulty: 🟡 Medium Topics: Array, String, Recursion Solution Problem Understanding The problem asks us to generate every strobogrammatic number of a given length n . A strobogrammatic number is a number that still appears valid after being rotated 180 degrees. Not every digit works under rotation. Only the following digit mappings are valid: Original Rotated 0 0 1 1 6 9 8 8 9 6 For example,...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 433 - Minimum Genetic Mutation

LeetCode Problem 433 Difficulty: 🟡 Medium Topics: Hash Table, String, Breadth-First Search Solution Problem Understanding The problem describes a mutation process between genetic sequences. Each gene is represented as a string of exactly 8 characters, and every character must be one of four possible nucleotides: 'A' , 'C' , 'G' , or 'T' . We are given three inputs: startGene , the initial gene sequence endGene , the target gene...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1573 - Number of Ways to Split a String

LeetCode Problem 1573 Difficulty: 🟡 Medium Topics: Math, String Solution LeetCode 1573 - Number of Ways to Split a String Problem Understanding The problem gives us a binary string s , meaning the string contains only the characters '0' and '1' . We must split this string into exactly three non-empty parts: s1 s2 s3 These three parts must concatenate back into the original string: s1 + s2 + s3...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1370 - Increasing Decreasing String

LeetCode Problem 1370 Difficulty: 🟢 Easy Topics: Hash Table, String, Counting Solution Problem Understanding The problem requires us to reorder a string s by repeatedly picking characters in a specific increasing and decreasing pattern. In other words, we need to simulate a process where, at each iteration, we first pick the smallest character and keep picking increasingly larger characters until we cannot pick anymore. Then, we switch directions: we pick...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1417 - Reformat The String

LeetCode Problem 1417 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem gives us an alphanumeric string s that contains only lowercase English letters and digits. Our task is to rearrange the characters so that no two adjacent characters belong to the same category. In other words, every letter must be next to a digit, and every digit must be next to a letter. The important detail is that...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1702 - Maximum Binary String After Change

LeetCode Problem 1702 Difficulty: 🟡 Medium Topics: String, Greedy Solution LeetCode 1702, Maximum Binary String After Change Problem Understanding This problem gives us a binary string consisting only of '0' and '1' . We are allowed to repeatedly apply two transformation rules: Replace "00" with "10" Replace "10" with "01" We may perform these operations as many times as we want, in any order, and the goal is to produce...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 734 - Sentence Similarity

LeetCode Problem 734 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding The problem gives us two sentences, where each sentence is represented as an array of words. We are also given a list of word pairs that define which words are considered similar. Our task is to determine whether the two sentences are similar according to the following rules: The sentences must contain the same number of...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 443 - String Compression

LeetCode Problem 443 Difficulty: 🟡 Medium Topics: Two Pointers, String Solution Problem Understanding The problem asks us to perform in place string compression on an array of characters. The input is not a string object, but a mutable array named chars , where each element is a single character. The compression rule is based on groups of consecutive repeated characters. For every contiguous block of identical characters: If the character...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1447 - Simplified Fractions

LeetCode Problem 1447 Difficulty: 🟡 Medium Topics: Math, String, Number Theory Solution Problem Understanding The problem asks us to generate all simplified fractions between 0 and 1 (exclusive) where the denominator does not exceed a given integer n . A simplified fraction is one in which the numerator and denominator are coprime , meaning their greatest common divisor (GCD) is 1. For example, 2/4 is not simplified because it reduces...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 10 - Regular Expression Matching

LeetCode Problem 10 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Recursion Solution Problem Understanding LeetCode 10, Regular Expression Matching, asks us to determine whether an entire input string s matches a pattern p . The pattern supports two special regular expression characters: . matches any single character * matches zero or more occurrences of the character immediately before it The important detail is that the match must cover the entire...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 125 - Valid Palindrome

LeetCode Problem 125 Difficulty: 🟢 Easy Topics: Two Pointers, String Solution Problem Understanding This problem asks us to determine whether a given string is a palindrome after applying two transformations: Convert all uppercase letters to lowercase. Remove all non-alphanumeric characters. An alphanumeric character is any English letter ( a-z , A-Z ) or digit ( 0-9 ). Characters such as spaces, punctuation marks, commas, colons, and symbols should be ignored...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1807 - Evaluate the Bracket Pairs of a String

LeetCode Problem 1807 Difficulty: 🟡 Medium Topics: Array, Hash Table, String Solution Problem Understanding The problem asks us to process a string s that contains bracketed keys like (name) or (age) and replace them with corresponding values provided in the knowledge array. Each element of knowledge is a two-element array [key, value] mapping a key to its replacement value. If a key in s is not present in knowledge ,...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 831 - Masking Personal Information

LeetCode Problem 831 Difficulty: 🟡 Medium Topics: String Solution Problem Understanding The problem requires creating a masked version of a personal information string s that can either be an email address or a phone number . The goal is to obscure sensitive information while keeping enough data to identify the user minimally. For emails , we need to lowercase all characters and replace the middle letters of the name (everything...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 761 - Special Binary String

LeetCode Problem 761 Difficulty: 🔴 Hard Topics: String, Divide and Conquer, Sorting Solution Problem Understanding The problem asks us to manipulate a special binary string to produce the lexicographically largest possible string. A special binary string has two properties: it contains equal numbers of 1 s and 0 s, and at every point in the string, the number of 1 s is at least as many as the number of...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1629 - Slowest Key

LeetCode Problem 1629 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding This problem asks us to determine which key on a keypad was pressed for the longest duration during a test sequence. We are given two pieces of information: a list of releaseTimes indicating when each key was released, and a string keysPressed representing the keys pressed in order. The first key is considered pressed at time 0, and...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 520 - Detect Capital

LeetCode Problem 520 Difficulty: 🟢 Easy Topics: String Solution only conditions are satisfied.

Practice › LeetCode › LeetCode 03xx ›

LeetCode 306 - Additive Number

LeetCode Problem 306 Difficulty: 🟡 Medium Topics: String, Backtracking Solution Problem Understanding The problem asks us to determine whether a given string of digits can be partitioned into a valid additive sequence. An additive sequence is a sequence of numbers where: There are at least three numbers. Every number after the first two is equal to the sum of the previous two numbers. No number is allowed to contain leading...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1416 - Restore The Array

LeetCode Problem 1416 Difficulty: 🔴 Hard Topics: String, Dynamic Programming Solution Problem Understanding The problem gives us a string s that represents several positive integers concatenated together without spaces. Originally, the array contained integers in the range [1, k] , and none of the integers had leading zeros. Our task is to determine how many different valid arrays could have produced the string s . For example, if: s =...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 214 - Shortest Palindrome

LeetCode Problem 214 Difficulty: 🔴 Hard Topics: String, Rolling Hash, String Matching, Hash Function Solution Problem Understanding The problem gives a string s and asks us to create the shortest possible palindrome by adding characters only at the beginning of the string. A palindrome is a string that reads the same forward and backward. The key restriction is that we are not allowed to insert characters in the middle or...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1255 - Maximum Score Words Formed by Letters

LeetCode Problem 1255 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Dynamic Programming, Backtracking, Bit Manipulation, Counting, Bitmask Solution Problem Understanding This problem asks us to select a subset of words that can be constructed using a limited supply of letters, such that the total score of all chosen words is maximized. We are given three inputs: words , an array of strings letters , a multiset of available characters...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1461 - Check If a String Contains All Binary Codes of Size K

LeetCode Problem 1461 Difficulty: 🟡 Medium Topics: Hash Table, String, Bit Manipulation, Rolling Hash, Hash Function Solution Problem Understanding The problem gives us a binary string s and an integer k . A binary string contains only the characters '0' and '1' . We must determine whether every possible binary code of length k appears somewhere inside s as a substring. A binary code of length k is simply any...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1160 - Find Words That Can Be Formed by Characters

LeetCode Problem 1160 Difficulty: 🟢 Easy Topics: Array, Hash Table, String, Counting Solution Problem Understanding This problem asks us to determine which words from a given list can be constructed using the characters available in another string, chars . A word is considered good if every character it needs exists in chars in sufficient quantity. Importantly, characters in chars can only be used once per word , meaning repeated letters...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 267 - Palindrome Permutation II

LeetCode Problem 267 Difficulty: 🟡 Medium Topics: Hash Table, String, Backtracking Solution Problem Understanding The problem asks us to generate every unique palindrome that can be formed by rearranging the characters of a given string. A palindrome is a string that reads the same forward and backward. For example, "abba" and "racecar" are palindromes. The task is not to determine whether the original string is already a palindrome. Instead, we...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 990 - Satisfiability of Equality Equations

LeetCode Problem 990 Difficulty: 🟡 Medium Topics: Array, String, Union-Find, Graph Theory Solution Problem Understanding The problem is asking us to determine if a set of equations between single-letter variables can all be satisfied simultaneously. Each equation is either of the form "xi==yi" or "xi!=yi" , where xi and yi are lowercase letters from 'a' to 'z' . The task is to assign an integer value to each variable such...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1111 - Maximum Nesting Depth of Two Valid Parentheses Strings

LeetCode Problem 1111 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem asks us to take a valid parentheses string seq and split it into two disjoint subsequences A and B such that each subsequence is itself a valid parentheses string (VPS). The goal is to assign each character in seq to either A or B in a way that minimizes the maximum nesting depth between the two...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1286 - Iterator for Combination

LeetCode Problem 1286 Difficulty: 🟡 Medium Topics: String, Backtracking, Design, Iterator Solution Problem Understanding The problem asks us to design a class CombinationIterator that generates all combinations of a given length from a string of sorted, distinct lowercase letters. The iterator must return these combinations in lexicographical order , meaning alphabetically. The input consists of two components: a string characters , which contains unique letters sorted in ascending order, and...

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LeetCode 241 - Different Ways to Add Parentheses

LeetCode Problem 241 Difficulty: 🟡 Medium Topics: Math, String, Dynamic Programming, Recursion, Memoization Solution Problem Understanding The problem gives us a mathematical expression as a string containing integers and arithmetic operators ( + , - , * ). We are asked to compute all possible results that can be obtained by inserting parentheses in every valid way. The key detail is that we are not evaluating the expression using normal...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1166 - Design File System

LeetCode Problem 1166 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Trie Solution Problem Understanding This problem asks us to design a simplified file system that supports two operations: Creating a new path with an associated integer value. Retrieving the value stored at a path. A path behaves similarly to a directory structure in a real operating system. Every path is composed of components separated by / . For example:...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 423 - Reconstruct Original Digits from English

LeetCode Problem 423 Difficulty: 🟡 Medium Topics: Hash Table, Math, String Solution Problem Understanding The problem asks us to reconstruct digits from a jumbled string of English letters representing numbers from 0 to 9 . The input string s contains only characters from the set ["e","g","f","i","h","o","n","s","r","u","t","w","v","x","z"] , which correspond to the letters used in English words for digits ( "zero" , "one" , "two" , etc.). The order of the...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 859 - Buddy Strings

LeetCode Problem 859 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem asks whether it is possible to make two strings equal by performing exactly one swap operation on the first string s . A swap operation means choosing two different indices i and j in s and exchanging the characters at those positions. After performing that single swap, the resulting string must match goal . The...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1616 - Split Two Strings to Make Palindrome

LeetCode Problem 1616 Difficulty: 🟡 Medium Topics: Two Pointers, String Solution Problem Understanding This problem gives us two strings, a and b , both having the same length. We are allowed to choose a split index and divide both strings at exactly the same position. If we split: a into aprefix + asuffix b into bprefix + bsuffix then we may form two candidate strings: aprefix + bsuffix bprefix +...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 981 - Time Based Key-Value Store

LeetCode Problem 981 Difficulty: 🟡 Medium Topics: Hash Table, String, Binary Search, Design Solution Problem Understanding The problem asks us to design a data structure that behaves like a key-value store, but with an important twist: the same key can have multiple values over time. Every value is associated with a timestamp, and later queries must return the most recent value that existed at or before a requested timestamp. In...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1169 - Invalid Transactions

LeetCode Problem 1169 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Sorting Solution Problem Understanding The problem asks us to identify transactions that are potentially invalid based on two rules. First, any transaction with an amount greater than $1000 is invalid. Second, any transaction is invalid if there exists another transaction with the same name that occurs within 60 minutes but in a different city. The input is a list...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 929 - Unique Email Addresses

LeetCode Problem 929 Difficulty: 🟢 Easy Topics: Array, Hash Table, String Solution Problem Understanding The problem gives us a list of email addresses and asks how many unique destinations actually receive emails after applying Gmail-like normalization rules. Each email consists of two parts separated by the '@' symbol: The local name, which appears before '@' The domain name, which appears after '@' The domain name is always preserved exactly as...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 332 - Reconstruct Itinerary

LeetCode Problem 332 Difficulty: 🔴 Hard Topics: Array, String, Depth-First Search, Graph Theory, Sorting, Heap (Priority Queue), Eulerian Circuit Solution Problem Understanding The problem is asking us to reconstruct a flight itinerary given a list of tickets. Each ticket is a one-way flight represented as a pair of airport codes [from, to] . The traveler always starts at "JFK" , and the itinerary must use all tickets exactly once ....

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1190 - Reverse Substrings Between Each Pair of Parentheses

LeetCode Problem 1190 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem gives us a string s containing lowercase English letters and balanced parentheses. Our task is to repeatedly reverse the substrings enclosed inside every matching pair of parentheses, starting from the innermost pair first. After all reversals are complete, we must return the final string without any parentheses. The important detail is that nested parentheses matter. When...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 388 - Longest Absolute File Path

LeetCode Problem 388 Difficulty: 🟡 Medium Topics: String, Stack, Depth-First Search Solution Problem Understanding The problem gives us a string representation of a file system hierarchy. Every line represents either a directory or a file. The hierarchy is encoded using newline characters ( \n ) to separate entries and tab characters ( \t ) to indicate nesting depth. A line with no leading tabs exists at depth 0 , meaning...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 830 - Positions of Large Groups

LeetCode Problem 830 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem asks us to identify all large groups in a given string and return their positions as intervals. A group is a sequence of consecutive identical characters. For example, in the string abbxxxxzzy , the groups are: "a" "bb" "xxxx" "zz" "y" Each group can be represented by its starting index and ending index , inclusive. For example,...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 556 - Next Greater Element III

LeetCode Problem 556 Difficulty: 🟡 Medium Topics: Math, Two Pointers, String Solution LeetCode 556 - Next Greater Element III Problem Understanding This problem asks us to find the smallest integer that is strictly greater than a given integer n , while using exactly the same digits as n . In other words, we are allowed to rearrange the digits of the number, but we cannot add or remove digits. Among...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1794 - Count Pairs of Equal Substrings With Minimum Difference

LeetCode Problem 1794 Difficulty: 🟡 Medium Topics: Hash Table, String, Greedy Solution Problem Understanding This problem asks us to count quadruples of indices (i, j, a, b) where substrings from two given strings are equal and the difference j - a is minimized. Specifically, i and j define a substring in firstString , while a and b define a substring in secondString . The substrings themselves must be identical, and...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1154 - Day of the Year

LeetCode Problem 1154 Difficulty: 🟢 Easy Topics: Math, String Solution Problem Understanding The problem asks us to compute the ordinal day number of a year given a date in the format YYYY-MM-DD . For example, January 1st is always day 1, January 2nd is day 2, and December 31st is either day 365 or 366 depending on whether the year is a leap year. The input is a string that...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1638 - Count Substrings That Differ by One Character

LeetCode Problem 1638 Difficulty: 🟡 Medium Topics: Hash Table, String, Dynamic Programming, Enumeration Solution Problem Understanding The problem asks us to count how many pairs of substrings, one taken from s and one taken from t , differ by exactly one character. A substring is contiguous, so for a string like "abcde" , valid substrings include "a" , "bc" , "abcd" , and "cde" . We are not allowed to...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1165 - Single-Row Keyboard

LeetCode Problem 1165 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem describes a single-row keyboard where each key is positioned at a unique index from 0 to 25. You are given a string keyboard of length 26 that specifies the layout of all lowercase English letters. The initial finger position is at index 0, and moving the finger from index i to index j takes |i...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 657 - Robot Return to Origin

LeetCode Problem 657 Difficulty: 🟢 Easy Topics: String, Simulation Solution Problem Understanding This problem asks us to determine whether a robot, starting at the origin (0, 0) on a 2D plane, returns to the origin after executing a sequence of moves. Each move is represented by a character in a string: 'R' moves right, 'L' moves left, 'U' moves up, and 'D' moves down. The robot always moves exactly one...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1758 - Minimum Changes To Make Alternating Binary String

LeetCode Problem 1758 Difficulty: 🟢 Easy Topics: String Solution Problem Understanding The problem gives us a binary string s , which means the string contains only the characters '0' and '1' . In one operation, we are allowed to flip a character, meaning we can change '0' into '1' or '1' into '0' . Our goal is to transform the string into an alternating binary string using the minimum number...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1307 - Verbal Arithmetic Puzzle

LeetCode Problem 1307 Difficulty: 🔴 Hard Topics: Array, Math, String, Backtracking Solution Problem Understanding The problem gives a collection of words on the left side of an equation and a single result word on the right side. Each distinct uppercase letter must be assigned a unique digit from 0 to 9 . After replacing every letter with its assigned digit, the numerical values represented by the left-side words must sum...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1773 - Count Items Matching a Rule

LeetCode Problem 1773 Difficulty: 🟢 Easy Topics: Array, String Solution Problem Understanding The problem asks us to determine how many items in a list satisfy a given rule. Each item is represented as a list of three strings: its type, color, and name. The rule is given as two strings: ruleKey and ruleValue . The ruleKey specifies which attribute of the item to compare ( type , color , or...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1496 - Path Crossing

LeetCode Problem 1496 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding This problem asks us to simulate movement on a 2D plane according to a string of directional instructions. Each character in the string represents a move of one unit in a cardinal direction: 'N' for north (up), 'S' for south (down), 'E' for east (right), and 'W' for west (left). You always start at the origin (0,...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1257 - Smallest Common Region

LeetCode Problem 1257 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Tree, Depth-First Search, Breadth-First Search Solution Problem Understanding This problem describes a hierarchical relationship between geographic regions. Each list in regions represents a parent-child relationship where the first element is the parent region and every remaining element in the list is directly contained within that parent. For example, the list: ["North America", "United States", "Canada"] means that "North America"...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1598 - Crawler Log Folder

LeetCode Problem 1598 Difficulty: 🟢 Easy Topics: Array, String, Stack Solution Problem Understanding In this problem, we are simulating navigation inside a file system. The system starts at the main folder, and we are given a sequence of folder navigation operations. Each operation changes the current directory in one of three ways. The operation "../" means moving up to the parent directory. However, if we are already at the main...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 692 - Top K Frequent Words

LeetCode Problem 692 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Trie, Sorting, Heap (Priority Queue), Bucket Sort, Counting Solution Problem Understanding The problem gives us an array of strings called words and an integer k . Each string represents a word, and words may appear multiple times in the array. Our task is to return the k most frequent words. The result must follow two ordering rules. First, words...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 402 - Remove K Digits

LeetCode Problem 402 Difficulty: 🟡 Medium Topics: String, Stack, Greedy, Monotonic Stack Solution Problem Understanding The problem gives us a numeric string num and an integer k . We must remove exactly k digits from the number so that the resulting integer is as small as possible. The key detail is that we are not allowed to reorder digits. We may only delete digits while preserving the relative order of...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 481 - Magical String

LeetCode Problem 481 Difficulty: 🟡 Medium Topics: Two Pointers, String Solution Problem Understanding This problem defines a special infinite sequence called the magical string. The string contains only the characters '1' and '2' , and its defining property is very unusual: If you group consecutive identical digits together and record the length of each group, the resulting sequence of lengths recreates the original string itself. For example, the beginning of...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 257 - Binary Tree Paths

LeetCode Problem 257 Difficulty: 🟢 Easy Topics: String, Backtracking, Tree, Depth-First Search, Binary Tree Solution Problem Understanding The problem asks us to return every path from the root of a binary tree to each leaf node. A root-to-leaf path is formed by starting at the root node and continuously moving downward through child nodes until reaching a node that has no children. The input is the root node of a...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 13 - Roman to Integer

LeetCode Problem 13 Difficulty: 🟢 Easy Topics: Hash Table, Math, String Solution Problem Understanding The problem gives a Roman numeral string and asks us to convert it into its corresponding integer value. Roman numerals use seven symbols: Symbol Value I 1 V 5 X 10 L 50 C 100 D 500 M 1000 Most Roman numerals follow a simple additive rule. Symbols are written from largest to smallest, and we...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 6 - Zigzag Conversion

LeetCode Problem 6 Difficulty: 🟡 Medium Topics: String Solution Problem Understanding The problem asks us to transform a string into a zigzag pattern across a fixed number of rows, then read the characters row by row to produce the final result. For example, if the input string is "PAYPALISHIRING" and the number of rows is 3 , the zigzag layout looks like this: P A H N A P L...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 205 - Isomorphic Strings

LeetCode Problem 205 Difficulty: 🟢 Easy Topics: Hash Table, String Solution Problem Understanding The problem asks us to determine whether two strings, s and t , are isomorphic . Two strings are considered isomorphic if there exists a one-to-one mapping between characters in s and characters in t such that replacing every character in s according to this mapping produces t . In simpler terms, each character in s must...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 3 - Longest Substring Without Repeating Characters

LeetCode Problem 3 Difficulty: 🟡 Medium Topics: Hash Table, String, Sliding Window Solution Longest Substring Without Repeating Characters Problem Understanding The problem gives a string s and asks for the length of the longest substring that contains no repeated characters. A substring is a continuous section of the string. This detail matters because characters must remain adjacent. For example, in "pwwkew" , the sequence "pwke" is not valid because those...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 5 - Longest Palindromic Substring

LeetCode Problem 5 Difficulty: 🟡 Medium Topics: Two Pointers, String, Dynamic Programming Solution Longest Palindromic Substring The problem asks us to find the longest substring of a given string that is also a palindrome. A palindrome is a sequence that reads the same forward and backward. The substring must be contiguous, which means the characters must appear next to each other in the original string. For example, in the string...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 208 - Implement Trie (Prefix Tree)

LeetCode Problem 208 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Trie Solution Problem Understanding The problem asks us to design and implement a Trie, also called a Prefix Tree. A Trie is a specialized tree structure for storing strings in a way that makes prefix-based operations very efficient. We need to implement three operations: insert(word) stores a word inside the Trie. search(word) checks whether the exact word exists in...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 8 - String to Integer (atoi)

LeetCode Problem 8 Difficulty: 🟡 Medium Topics: String Solution Problem Understanding The problem asks us to implement a simplified version of the C/C++ atoi function, which converts a string into a 32-bit signed integer. The conversion process is strict and follows several rules in a specific order. The function must parse the string carefully and stop as soon as the input becomes invalid for integer parsing. The input is a...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1028: Recover a Tree From Preorder Traversal

Problem Restatement We run a preorder depth-first traversal on a binary tree. At each node, we output its depth (as a count of dashes) followed by its value. For example, a tree with root 1 and left child 2 and right child 5 would produce "1-2--3--4-5--6--7" for a specific tree. We are given this string traversal and need to recover the original binary tree. The official constraints state that the...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1021: Remove Outermost Parentheses

Problem Restatement A valid parentheses string is either empty, a concatenation of valid strings, or a valid string wrapped in parentheses. A primitive valid parentheses string is one that cannot be split into two non-empty valid strings. We are given a valid parentheses string s . We need to remove the outermost parentheses of every primitive part in s and return the result. The official constraints state that 1 <=...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1016: Binary String With Substrings Representing 1 To N

Problem Restatement We are given a binary string s and a positive integer n . We need to return true if every integer from 1 to n can be represented as a substring of s when the substring is interpreted as a binary number. The official constraints state that 1 <= s.length <= 1000 and 1 <= n <= 10^9 . Input and Output Item Meaning Input Binary string s...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1003: Check If Word Is Valid After Substitutions

Problem Restatement We are given a string s . A string is valid if we can repeatedly remove the substring "abc" from it until the string becomes empty. We need to return true if s is valid, and false otherwise. The official constraints state that 1 <= s.length <= 2 * 10^4 and s consists only of letters a , b , and c . Input and Output Item Meaning...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1062: Longest Repeating Substring

Problem Restatement We are given a string s . Return the length of the longest repeating substring (a substring that appears at least twice). If no repeating substring exists, return 0 . The official constraints state that 1 <= s.length <= 2000 and s consists only of lowercase English letters. Input and Output Item Meaning Input String s Output Length of the longest repeating substring Function shape: def longestRepeatingSubstring(s: str)...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1078: Occurrences After Bigram

Problem Restatement We are given a string text and two words first and second . We need to return all words that come immediately after the bigram first second in text . The official constraints state that 1 <= text.length <= 1000 and text consists only of lowercase English letters and spaces. Input and Output Item Meaning Input String text , words first and second Output List of words immediately...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1055: Shortest Way to Form String

Problem Restatement A subsequence of a string is formed by deleting some (or no) characters without changing the order. We are given two strings source and target . We need to form target by choosing the minimum number of subsequences of source and concatenating them. Return the minimum number of subsequences needed, or -1 if it is impossible. The official constraints state that 1 <= source.length, target.length <= 1000 ....

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1079: Letter Tile Possibilities

Problem Restatement We have a set of tiles with letters on them. We need to return the number of possible non-empty sequences of letters using some or all of the tiles. Two sequences are the same if they use the same letters in the same order. The official constraints state that 1 <= tiles.length <= 7 and each tiles[i] is an uppercase English letter. Input and Output Item Meaning Input...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1100: Find K-Length Substrings With No Repeated Characters

Problem Restatement We are given a string s and an integer k . Return the number of substrings of length k with no repeated characters. The official constraints state that 1 <= s.length <= 10^4 and 1 <= k <= 26 . Input and Output Item Meaning Input String s and integer k Output Count of length- k substrings with all distinct characters Function shape: def numKLenSubstrNoRepeats(s: str, k: int)...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1032: Stream of Characters

Problem Restatement We implement a StreamChecker class: StreamChecker(words) : Initialize with a list of words. query(letter) : Add the letter to the stream. Return true if any word in words is a suffix of the letters queried so far. The official constraints state that 1 <= words.length <= 2000 , 1 <= words[i].length <= 2000 , and up to 4 * 10^4 calls to query . Input and Output Item...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1002: Find Common Characters

Problem Restatement We are given an array of strings words . We need to return a list of all characters that appear in every string in words , including duplicates. If a character appears k times in every string, we include it k times in the answer. The answer can be returned in any order. The official constraints state that 1 <= words.length <= 100 , 1 <= words[i].length <=...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1048: Longest String Chain

Problem Restatement We are given an array of words. A word wordA is a predecessor of wordB if we can insert exactly one letter anywhere in wordA to make it equal to wordB . A word chain is a sequence of words [word_1, word_2, ..., word_k] where each word_i is a predecessor of word_{i+1} . Return the length of the longest word chain. The official constraints state that 1 <=...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1041: Robot Bounded In Circle

Problem Restatement A robot starts at the origin facing north and follows a sequence of instructions: 'G' : move one step forward. 'L' : turn left 90 degrees. 'R' : turn right 90 degrees. We need to determine if the robot stays within a bounded circle (i.e., the robot's path is bounded) when the instructions are repeated indefinitely. The official constraints state that 1 <= instructions.length <= 100 . Input...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1061: Lexicographically Smallest Equivalent String

Problem Restatement We are given two strings s1 and s2 of the same length, and a string baseStr . From s1 and s2 , we define equivalence classes: s1[i] and s2[i] are equivalent for each i . The equivalence relation is reflexive, symmetric, and transitive. We need to return the lexicographically smallest equivalent string of baseStr using the equivalence classes. The official constraints state that 1 <= s1.length == s2.length...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1044: Longest Duplicate Substring

Problem Restatement We are given a string s . We need to return the longest substring that appears at least twice. If no such substring exists, return an empty string. The official constraints state that 2 <= s.length <= 3 * 10^4 and s consists only of lowercase English letters. Input and Output Item Meaning Input String s Output Longest duplicate substring (any valid answer accepted) Function shape: def longestDupSubstring(s:...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1081: Smallest Subsequence of Distinct Characters

Problem Restatement We are given a string s . We need to return the lexicographically smallest subsequence of s that contains all distinct characters of s exactly once. The official constraints state that 1 <= s.length <= 1000 and s consists of lowercase English letters. Note: This problem is identical to LeetCode 316 (Remove Duplicate Letters). Input and Output Item Meaning Input String s Output Lexicographically smallest subsequence with each...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1087: Brace Expansion

Problem Restatement We are given a string s representing a word with the following format: A single letter c means the set {c} . A brace expression {a,b,c,...} means choose one of the letters. Consecutive groups are concatenated. Return all strings generated by the pattern in sorted order. The official constraints state that 1 <= s.length <= 50 . Input and Output Item Meaning Input Pattern string s Output Sorted...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1071: Greatest Common Divisor of Strings

Problem Restatement We say string t divides string s if s is formed by concatenating t one or more times. We are given two strings str1 and str2 . Return the largest string t that divides both str1 and str2 . The official constraints state that 1 <= str1.length, str2.length <= 1000 . Input and Output Item Meaning Input Strings str1 and str2 Output Largest common divisor string Function shape:...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1023: Camelcase Matching

Problem Restatement We are given an array of strings queries and a string pattern . A query string matches the pattern if we can insert lowercase letters into the pattern to make it equal to the query. In other words, the pattern is a subsequence of the query, and all uppercase letters in the query must match uppercase letters in the pattern. Return a boolean array where result[i] is true...

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LeetCode 1096: Brace Expansion II

Problem Restatement We are given a string expression representing a set of strings under a brace expansion language: A letter c represents the set {c} . {e1, e2, ...} represents the union of the sets. e1 e2 (concatenation) represents all strings formed by concatenating one from each. Return all strings in the resulting set in sorted order. The official constraints state that 1 <= expression.length <= 60 . Input and...

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LeetCode 1065: Index Pairs of a String

Problem Restatement We are given a string text and a list of strings words . Return all [i, j] pairs such that the substring text[i:j+1] is in words . The result must be sorted by i , then by j . The official constraints state that 1 <= text.length <= 100 and 1 <= words.length <= 20 . Input and Output Item Meaning Input String text and list words Output...

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LeetCode 1047: Remove All Adjacent Duplicates In String

Problem Restatement We are given a string s . In one step, we choose two adjacent equal letters and remove them. We repeat this until no adjacent equal letters remain. Return the final string. The result is always unique regardless of the order of removals. The official constraints state that 1 <= s.length <= 10^5 and s consists only of lowercase English letters. Input and Output Item Meaning Input String...

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LeetCode 1092: Shortest Common Supersequence

Problem Restatement We are given two strings str1 and str2 . We need to return the shortest string such that both str1 and str2 are subsequences of it. If there are multiple answers, return any. The official constraints state that 1 <= str1.length, str2.length <= 1000 . Input and Output Item Meaning Input Strings str1 and str2 Output Shortest common supersequence Function shape: def shortestCommonSupersequence(str1: str, str2: str) -> str:...

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LeetCode 925: Long Pressed Name

Problem Restatement We are given two strings: Item Meaning name The name the friend intended to type typed The actual string produced by the keyboard Sometimes a key may be long pressed. If the intended character is "a" , the typed output may contain "a" , "aa" , "aaa" , and so on. We need to return true if typed could be produced from name by long pressing some characters....

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LeetCode 848: Shifting Letters

Problem Restatement We are given a lowercase string s and an integer array shifts of the same length. A shift moves a character to the next letter in the alphabet, wrapping around from "z" to "a" . For each shifts[i] = x , we shift the first i + 1 letters of s by x positions. Return the final string after all shifts are applied. Input and Output Item Meaning...

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LeetCode 972: Equal Rational Numbers

Problem Restatement We are given two strings s and t . Each string represents a non-negative rational number. A number may be written in one of these forms: "123" "123.456" "123.456(789)" Parentheses mark the repeating decimal part. Return true if both strings represent the same rational number. Otherwise, return false . Examples of equivalent forms: "0.1(6)" == "0.1666(6)" "0.9(9)" == "1." The official note says the integer part length is...

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LeetCode 899: Orderly Queue

Problem Restatement We are given a string s and an integer k . In one operation: Choose one of the first k characters of s . Remove it from its current position. Append it to the end of the string. Return the lexicographically smallest string we can get after any number of operations. Input and Output Item Meaning Input String s Input Integer k Output Lexicographically smallest reachable string Operation...

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LeetCode 921: Minimum Add to Make Parentheses Valid

Problem Restatement We are given a string s containing only parentheses: "(" ")" In one move, we may insert one parenthesis at any position. Return the minimum number of insertions needed to make the string valid. A valid parentheses string is one where every opening parenthesis has a matching closing parenthesis in the correct order. The official statement defines the operation as inserting one parenthesis anywhere in the string. Input...

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LeetCode 844: Backspace String Compare

Problem Restatement We are given two strings s and t . Each string contains lowercase letters and the character "#" . The "#" character acts like a backspace in a text editor. It deletes the character immediately before it. If there is no character before it, it does nothing. Return True if both strings become equal after applying all backspaces. Otherwise, return False . Input and Output Item Meaning Input...

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LeetCode 949: Largest Time for Given Digits

Problem Restatement We are given an array arr containing exactly four digits. We need to use each digit exactly once to form a valid 24-hour time in this format: HH:MM The hour must be between: 00 and 23 The minute must be between: 00 and 59 Return the latest valid time that can be made. If no valid time can be made, return an empty string. The official statement defines...

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LeetCode 843: Guess the Word

Problem Restatement We are given a list of unique strings words . Each word has length 6 . One word from the list is chosen as the secret word. We can call: master.guess(word) This returns: Return value Meaning -1 The guessed word is not in the word list 0 to 6 Number of positions where the guessed word matches the secret word We must find the secret word within at...

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LeetCode 824: Goat Latin

Problem Restatement We are given a sentence made of words separated by single spaces. Each word contains only uppercase and lowercase English letters. We need to convert every word into Goat Latin using these rules: If a word starts with a vowel, append "ma" . If a word starts with a consonant, move the first letter to the end, then append "ma" . Append 'a' characters based on the word...

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LeetCode 842: Split Array into Fibonacci Sequence

Problem Restatement We are given a string num containing only digits. We need to split it into a Fibonacci-like sequence. A valid Fibonacci-like sequence must satisfy these rules: Rule Meaning Length The sequence must contain at least 3 numbers Value limit Every number must fit in a 32-bit signed integer Fibonacci rule seq[i] + seq[i + 1] == seq[i + 2] Leading zero rule A number cannot have extra leading...

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LeetCode 966: Vowel Spellchecker

Problem Restatement We are given: wordlist queries For each query, return the corrected word according to these priority rules: If the query exactly matches a word in wordlist , return the query itself. Otherwise, if the query matches a word ignoring capitalization, return the first such word from wordlist . Otherwise, if the query matches a word after treating all vowels as interchangeable, return the first such word from wordlist...

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LeetCode 917: Reverse Only Letters

Problem Restatement We are given a string s . Reverse only the English letters in the string. All characters that are not English letters must stay at the same index. Both lowercase and uppercase English letters are reversed together. For example: s = "ab-cd" The letters are: a, b, c, d After reversing the letters: d, c, b, a The hyphen stays in the same position. Output: "dc-ba" Input and...

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LeetCode 821: Shortest Distance to a Character

Problem Restatement We are given a string s and a character c . The character c appears at least once in s . For every index i , we need to find the shortest distance from i to any index where s[index] == c . The distance between two indices i and j is: abs(i - j) Return an array answer , where: answer[i] = distance from i to the...

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LeetCode 916: Word Subsets

Problem Restatement We are given two string arrays: words1 words2 A string b is a subset of string a if every letter in b appears in a with at least the same frequency. For example: "wrr" is a subset of: "warrior" because "warrior" has at least one "w" and at least two "r" characters. But "wrr" is not a subset of: "world" because "world" has only one "r" . A...

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LeetCode 800: Similar RGB Color

Problem Restatement We are given a color string in this format: "#ABCDEF" The color has three hexadecimal components: Component Meaning AB Red CD Green EF Blue A color has shorthand form if each component has two equal hexadecimal digits. For example: "#AABBCC" can be written as: "#ABC" because: AA -> A BB -> B CC -> C We need to return a 7-character color string that has shorthand form and...

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LeetCode 820: Short Encoding of Words

Problem Restatement We are given an array of words. A valid encoding uses: Item Meaning s A reference string ending with # indices Starting positions inside s For each word, if we start reading from its index in s and stop before the next # , we must recover that word. For example: s = "time#bell#" indices = [0, 2, 5] This encodes: "time" from index 0 "me" from index...

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LeetCode 839: Similar String Groups

Problem Restatement We are given an array of strings strs . Every string is an anagram of every other string in the array, and all strings have the same length. Two strings are similar if either: They are already equal. We can make them equal by swapping at most two letters in one of the strings. We need to return the number of similar string groups. Similarity is transitive for...

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LeetCode 944: Delete Columns to Make Sorted

Problem Restatement We are given an array of equal-length strings: strs Think of the strings as rows of a character matrix. We may delete some columns. A column is valid if its characters are in non-decreasing order from top to bottom. For a column c , we need: strs[0][c] <= strs[1][c] <= strs[2][c] <= ... Return the minimum number of columns that must be deleted so that every remaining column...

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LeetCode 819: Most Common Word

Problem Restatement We are given a string paragraph and a list of banned words. We need to return the most frequent word in paragraph that is not banned. Words are case-insensitive, so: "Bob" "bob" "BOB" all count as the same word. The answer must be returned in lowercase. Punctuation should be ignored. The paragraph may contain spaces and symbols such as: !?',;. The problem guarantees that at least one non-banned...

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LeetCode 893: Groups of Special-Equivalent Strings

Problem Restatement We are given an array words . All strings have the same length. In one move, we can swap two characters only if their indices have the same parity: Swap type Allowed? Even index with even index Yes Odd index with odd index Yes Even index with odd index No Two strings are special-equivalent if one can be changed into the other using any number of these moves....

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LeetCode 943: Find the Shortest Superstring

Problem Restatement We are given an array of strings: words We need to return the shortest string that contains every string in words as a substring. If several shortest answers exist, we may return any of them. The problem also says that no string in words is a substring of another string in words . This lets us focus on ordering and overlapping the strings, instead of removing already-contained strings....

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LeetCode 796: Rotate String

Problem Restatement We are given two strings: s goal A shift on s means moving the leftmost character to the rightmost position. For example: abcde -> bcdea We need to return True if s can become goal after some number of shifts. Otherwise, return False . Zero shifts are allowed, so if s == goal , the answer is True . Input and Output Item Meaning Input Two strings s...

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LeetCode 772: Basic Calculator III

Problem Restatement We are given a valid arithmetic expression string s . The expression may contain: non-negative integers + - * / ( ) spaces We need to evaluate the expression and return the integer result. The expression follows normal arithmetic rules: Parentheses are evaluated first. Multiplication and division happen before addition and subtraction. Operators with the same precedence are evaluated from left to right. Integer division truncates toward zero....

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LeetCode 722: Remove Comments

Problem Restatement We are given a C++ program as a list of strings: source Each string is one line of code. We need to remove two kinds of comments: Comment Type Meaning Line comment // Ignore // and everything after it on the same line Block comment /* ... */ Ignore everything from /* until the next non-overlapping */ After removing comments, return the remaining source code as a list...

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LeetCode 942: DI String Match

Problem Restatement We are given a string s of length n . Each character is either: "I" or: "D" We need to return any permutation perm of the integers from 0 to n such that: Character Required relation s[i] == "I" perm[i] < perm[i + 1] s[i] == "D" perm[i] > perm[i + 1] If multiple answers exist, any valid answer is accepted. The official statement defines the same reconstruction...

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LeetCode 771: Jewels and Stones

Problem Restatement We are given two strings: jewels stones Each character in jewels represents a type of stone that is a jewel. Each character in stones represents one stone we have. We need to count how many stones are also jewels. Letters are case-sensitive, so "a" and "A" are different stone types. The official statement also says all characters in jewels are unique. Input and Output Item Meaning Input jewels...

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LeetCode 721: Accounts Merge

Problem Restatement We are given a list of accounts. Each account has this format: [name, email1, email2, ...] Two accounts belong to the same person if they share at least one email address. After merging accounts, each returned account should contain: The person's name. All unique emails belonging to that person, sorted lexicographically. The returned accounts can be in any order. Accounts with the same name do not automatically belong...

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LeetCode 748: Shortest Completing Word

Problem Restatement We are given a string licensePlate and a list of words. A word is called a completing word if it contains every letter from licensePlate . When reading licensePlate : Ignore digits. Ignore spaces. Treat uppercase and lowercase letters as the same. If a letter appears multiple times, the word must contain it at least that many times. We need to return the shortest completing word from words...

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LeetCode 990: Satisfiability of Equality Equations

Problem Restatement We are given an array of equations. Each equation has exactly four characters and is one of two forms: "a==b" "a!=b" Each variable is a lowercase English letter. We need to return True if it is possible to assign integer values to the variables so that all equations are satisfied. Otherwise, return False . For example: ["a==b", "b!=a"] cannot be satisfied, because the first equation says a and...

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LeetCode 816: Ambiguous Coordinates

Problem Restatement We are given a string s that represents a coordinate pair after removing commas, spaces, and decimal points. For example: "(1, 3)" -> "(13)" "(2, 0.5)" -> "(205)" The string still contains the opening and closing parentheses. We need to return all possible original coordinate strings. The original coordinate representation had no unnecessary zeroes. So values like these are invalid: "00" "001" "0.0" "1.0" ".1" The final answer...

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LeetCode 770: Basic Calculator IV

Problem Restatement We are given an algebraic expression as a string. The expression may contain: integers variables + - * parentheses We are also given some variable values through: evalvars evalints For example: evalvars = ["e"] evalints = [1] means: e = 1 We need to simplify the expression after substituting known variables. The result should be returned as a list of string terms. The official format requires variable factors...

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LeetCode 720: Longest Word in Dictionary

Problem Restatement We are given an array of strings: words We need to find the longest word that can be built one character at a time by other words in the array. A word is valid if every prefix formed by removing the last character also exists in the array. For example: "world" is valid only if all these words exist: "w" "wo" "wor" "worl" "world" If there are multiple...

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LeetCode 49: Group Anagrams

Problem Restatement We are given an array of strings strs . We need to group strings that are anagrams of each other. Two strings are anagrams if they contain the same characters with the same frequencies, but possibly in a different order. The answer can be returned in any order. LeetCode states the input strings contain lowercase English letters, with 1 <= strs.length <= 10^4 and 0 <= strs[i].length <=...

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LeetCode 890: Find and Replace Pattern

Problem Restatement We are given a list of strings words and a string pattern . We need to return all words that match the pattern. A word matches the pattern if there is a bijection between letters in pattern and letters in the word. That means: Each pattern character maps to exactly one word character. No two pattern characters map to the same word character. The mapping must be consistent...

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LeetCode 940: Distinct Subsequences II

Problem Restatement We are given a string: s We need to count how many distinct non-empty subsequences exist. A subsequence is formed by deleting zero or more characters without changing the order of the remaining characters. For example, from: "abc" the subsequences include: "a" "b" "c" "ab" "ac" "bc" "abc" The empty subsequence does not count. Since the answer can become large, return it modulo: 10**9 + 7 The official...

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LeetCode 249: Group Shifted Strings

Problem Restatement We are given an array of lowercase strings. We need to group strings that belong to the same shifting sequence. A shift means changing every character to the next character in the alphabet: a -> b b -> c ... z -> a For example: "abc" -> "bcd" -> "cde" -> ... So these strings belong to the same group: "abc", "bcd", "xyz" because each one can be...

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LeetCode 248: Strobogrammatic Number III

Problem Restatement We are given two strings, low and high , representing two integers. Return the number of strobogrammatic numbers in the inclusive range: low <= num <= high A strobogrammatic number looks the same after being rotated 180 degrees. The numbers are represented as strings because the range can be large. The constraints say 1 <= low.length, high.length <= 15 , low and high contain only digits, low <=...

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LeetCode 247: Strobogrammatic Number II

Problem Restatement We are given an integer n . Return all strobogrammatic numbers of length n . A strobogrammatic number is a number that looks the same after being rotated 180 degrees. The answer may be returned in any order. The constraint is 1 <= n <= 14 . The valid rotated digit pairs are: Left Digit Right Digit 0 0 1 1 6 9 8 8 9 6 But...

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LeetCode 745: Prefix and Suffix Search

Problem Restatement We are given a list of words. We need to design a class: WordFilter that supports queries: f(prefix, suffix) The query should return the largest index of a word that: Starts with prefix Ends with suffix If no such word exists, return: -1 The official problem asks for the maximum index satisfying both prefix and suffix constraints. Input and Output Item Meaning Constructor WordFilter(words) Query f(pref, suff) Output...

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LeetCode 246: Strobogrammatic Number

Problem Restatement We are given a string num that represents an integer. We need to return true if the number looks the same after being rotated 180 degrees. Otherwise, return false . Only some digits are valid after rotation: Digit Rotates To 0 0 1 1 6 9 8 8 9 6 Digits like 2 , 3 , 4 , 5 , and 7 cannot appear in a valid rotated...

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LeetCode 792: Number of Matching Subsequences

Problem Restatement We are given a string s and an array of strings words . We need to return how many strings in words are subsequences of s . A subsequence is formed by deleting zero or more characters from a string without changing the order of the remaining characters. For example, "ace" is a subsequence of "abcde" . Input and Output Item Meaning Input A string s and a...

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LeetCode 245: Shortest Word Distance III

Problem Restatement We are given a list of words called wordsDict and two words, word1 and word2 . We need to return the shortest distance between the two words in the list. The distance between two words is the absolute difference between their indices. Unlike the previous problem, word1 and word2 may be the same word. If they are the same, we must use two different occurrences. Input and Output...

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LeetCode 44: Wildcard Matching

Problem Restatement We are given an input string s and a pattern p . The pattern may contain two special wildcard characters: Character Meaning ? Matches any single character * Matches any sequence of characters, including the empty sequence The match must cover the entire input string, not just part of it. The official problem states these wildcard rules directly. For example: s = "aa" p = "a" This returns:...

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LeetCode 244: Shortest Word Distance II

Problem Restatement We need to design a class called WordDistance . The class receives a list of words in the constructor. Then it must support a method called shortest(word1, word2) . This method returns the shortest index distance between word1 and word2 in the original list. The important detail is that shortest will be called many times with different pairs of words. So we should avoid scanning the whole word...

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LeetCode 43: Multiply Strings

Problem Restatement We are given two non-negative integers num1 and num2 , but they are stored as strings. We need to return their product, also as a string. We cannot use a built-in big integer library, and we cannot directly convert the whole input strings into integers. The official problem states this string multiplication requirement and restriction directly. Example: num1 = "2" num2 = "3" The product is: "6" Another...

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LeetCode 243: Shortest Word Distance

Problem Restatement We are given a list of words, wordsDict , and two different words, word1 and word2 . Both words appear in the list. We need to return the shortest distance between any occurrence of word1 and any occurrence of word2 . The distance between two words is the absolute difference between their indices. For example, if word1 appears at index i and word2 appears at index j ,...

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LeetCode 791: Custom Sort String

Problem Restatement We are given two strings: order s All characters in order are unique. The string order defines a custom ordering of characters. We need to rearrange the characters of s so that characters appearing in order follow that same relative order. Characters in s that do not appear in order may be placed anywhere in the result. Return any valid permutation of s . Input and Output Item...

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LeetCode 767: Reorganize String

Problem Restatement We are given a string s . We need to rearrange its characters so that no two adjacent characters are the same. Return any valid rearrangement. If no valid rearrangement exists, return an empty string. The problem allows any correct answer, not one specific output. The official statement says to rearrange the characters so adjacent characters are not equal, or return "" if impossible. Input and Output Item...

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LeetCode 838: Push Dominoes

Problem Restatement We are given a string dominoes . Each character describes the initial state of one domino: Character Meaning "L" Domino is pushed to the left "R" Domino is pushed to the right "." Domino is standing upright After each second, a falling domino pushes the adjacent upright domino in the same direction. If a standing domino receives equal force from both sides at the same time, it stays...

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LeetCode 988: Smallest String Starting From Leaf

Problem Restatement We are given the root of a binary tree. Each node has a value from 0 to 25 . Each value maps to a lowercase English letter: Value Letter 0 "a" 1 "b" 2 "c" ... ... 25 "z" We need to return the lexicographically smallest string that starts at a leaf and ends at the root. A leaf is a node with no children. The number of...

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LeetCode 788: Rotated Digits

Problem Restatement We are given an integer n . We need to count how many integers x in the range: 1 <= x <= n are good numbers. A number is good if, after rotating every digit by 180 degrees, we get: A valid number. A different number from the original. The valid digit rotations are: Digit Rotates to 0 0 1 1 2 5 5 2 6 9 8...

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LeetCode 712: Minimum ASCII Delete Sum for Two Strings

Problem Restatement We are given two strings: s1 s2 We may delete characters from either string. Deleting a character costs its ASCII value. We need to make the two strings equal with the minimum total deletion cost. Return that minimum cost. For example, deleting "s" costs: ord("s") = 115 The strings contain lowercase English letters, and their lengths are between 1 and 1000 . The official problem asks for the...

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LeetCode 960: Delete Columns to Make Sorted III

Problem Restatement We are given an array strs of equal-length strings. We may delete any set of column indices. When we delete a column, that position is removed from every string. After deletions, every remaining row must be lexicographically sorted by itself: strs[row][0] <= strs[row][1] <= strs[row][2] <= ... Return the minimum number of deleted columns. Important: the array of strings does not need to be sorted. Each individual row...

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LeetCode 763: Partition Labels

Problem Restatement We are given a string s . We need to split it into as many parts as possible so that each letter appears in at most one part. After concatenating all parts in order, we must get the original string back. Return the sizes of the parts. Input and Output Item Meaning Input A string s Output A list of partition sizes Goal Make as many partitions as...

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LeetCode 937: Reorder Data in Log Files

Problem Restatement We are given a list of log strings. Each log has this structure: identifier content The first word is the identifier. The rest of the log is the content. There are two kinds of logs: Log type Meaning Letter-log Every word after the identifier contains lowercase letters Digit-log Every word after the identifier contains digits We need to reorder the logs using these rules: All letter-logs come before...

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LeetCode 649: Dota2 Senate

Problem Restatement We are given a string senate . Each character represents one senator: Character Party "R" Radiant "D" Dire The voting process happens in rounds from left to right. On a senator's turn, if they still have voting rights, they can ban one senator from the opposite party. A banned senator loses voting rights for the current and all future rounds. If all remaining senators with voting rights belong...

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LeetCode 936: Stamping The Sequence

Problem Restatement We are given two strings: stamp target Initially, we start with a string made entirely of question marks: "?????" with the same length as target . In one move, we may place the stamp string over any position and overwrite the characters. For example: stamp = "abc" before = "?????" after = "??abc" The goal is to build target . We must return a sequence of stamping positions...

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LeetCode 737: Sentence Similarity II

Problem Restatement We are given two sentences represented as arrays of words: sentence1 sentence2 We are also given a list of similar word pairs: similarPairs Each pair: [x, y] means word x is similar to word y . Two sentences are similar if: They have the same number of words. For every index i , sentence1[i] is similar to sentence2[i] . A word is always similar to itself. Similarity is...

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LeetCode 648: Replace Words

Problem Restatement We are given a dictionary of root words and a sentence . A root can form a longer derivative word. For example: root = "cat" derivative = "cattle" Since "cattle" starts with "cat" , we can replace "cattle" with "cat" . For every word in the sentence, if it starts with one or more roots from the dictionary, replace it with the shortest matching root. If no root...

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LeetCode 696: Count Binary Substrings

Problem Restatement We are given a binary string s . We need to count non-empty substrings that satisfy both rules: Rule Meaning Equal counts The substring has the same number of 0 s and 1 s Grouped characters All 0 s are consecutive and all 1 s are consecutive Substrings that appear multiple times are counted multiple times. For example, "00110011" has two separate occurrences of "0011" and two separate...

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LeetCode 647: Palindromic Substrings

Problem Restatement We are given a string s . We need to count how many substrings of s are palindromes. A palindrome reads the same forward and backward. A substring is a contiguous part of the string. The same text can be counted multiple times if it appears at different positions. For example, in "aaa" , each "a" at each index is counted separately. The constraints are 1 <= s.length...

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LeetCode 811: Subdomain Visit Count

Problem Restatement We are given a list of count-paired domains. Each item has this format: "count domain" For example: "9001 discuss.leetcode.com" This means discuss.leetcode.com was visited 9001 times. A visit to a domain also counts as a visit to all of its parent subdomains: discuss.leetcode.com leetcode.com com Return a list of count-paired domains showing the total visit count for every domain and subdomain. The result may be returned in any...

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LeetCode 761: Special Binary String

Problem Restatement A special binary string is a binary string with two properties: It has the same number of 1 s and 0 s. Every prefix has at least as many 1 s as 0 s. We are given a special binary string s . One move lets us choose two consecutive non-empty special substrings and swap them. We need to return the lexicographically largest string possible after applying any...

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LeetCode 736: Parse Lisp Expression

Problem Restatement We are given a string expression that represents a Lisp-like expression. We need to evaluate it and return its integer value. An expression can be one of these forms: Form Meaning Integer A positive or negative integer Variable A name whose value comes from a surrounding let add Adds two expressions mult Multiplies two expressions let Assigns variables, then evaluates a final expression The official syntax includes integer...

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LeetCode 709: To Lower Case

Problem Restatement We are given a string s . Return a new string after replacing every uppercase letter with the same lowercase letter. All other characters stay unchanged. The string consists of printable ASCII characters. Input and Output Item Meaning Input A string s Output The same string with uppercase letters converted to lowercase Constraint 1 <= s.length <= 100 Characters Printable ASCII characters The function shape is: class Solution:...

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LeetCode 884: Uncommon Words from Two Sentences

Problem Restatement We are given two sentences, s1 and s2 . A word is uncommon if it satisfies both conditions: It appears exactly once in one sentence. It does not appear in the other sentence. Return all uncommon words. The answer may be returned in any order. LeetCode defines each sentence as lowercase words separated by single spaces. Input and Output Item Meaning Input Two strings s1 and s2 Output...

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LeetCode 984: String Without AAA or BBB

Problem Restatement We are given two integers a and b . We need to return any string s such that: Requirement Meaning Length s.length == a + b Count of a s contains exactly a letters 'a' Count of b s contains exactly b letters 'b' Forbidden substring s must not contain "aaa" Forbidden substring s must not contain "bbb" The problem guarantees that a valid answer exists. The constraints...

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LeetCode 692: Top K Frequent Words

Problem Restatement We are given a list of words and an integer k . We need to return the k most frequent words. The ordering rules are: Rule Meaning Higher frequency first More common words come earlier Lexicographical tie-break If two words have the same frequency, the smaller alphabetical word comes first The official statement defines lexicographical order using normal alphabetical string comparison. Input and Output Item Meaning Input words...

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LeetCode 691: Stickers to Spell Word

Problem Restatement We are given a list of sticker words and a target string. Each sticker contains lowercase English letters. We may cut letters from stickers and rearrange them to form the target. We have infinite copies of every sticker, so the same sticker type can be used more than once. Return the minimum number of stickers needed to form target . If it is impossible, return -1 . The...

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LeetCode 784: Letter Case Permutation

Problem Restatement We are given a string s . Each alphabet letter may be changed independently to lowercase or uppercase. Digits must stay the same. Return all possible strings we can create. The output may be returned in any order. Input and Output Item Meaning Input A string s Output A list of all possible letter-case permutations Letters Can become lowercase or uppercase Digits Must stay unchanged Constraint 1 <=...

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LeetCode 833: Find And Replace in String

Problem Restatement We are given a string s and three arrays: Array Meaning indices Starting positions where replacement may happen sources Source strings that must match at those positions targets Replacement strings For each operation i , we check whether sources[i] appears in the original string s starting at index indices[i] . If it matches, we replace that source substring with targets[i] . If it does not match, we do...

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LeetCode 809: Expressive Words

Problem Restatement We are given a string s and an array of query strings words . A query word is stretchy if it can be expanded to become exactly s . Expansion works on groups of the same consecutive character. For example, in: "heeellooo" the groups are: "h", "eee", "ll", "ooo" A group in the query word may be stretched only if the corresponding group in s has length at...

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LeetCode 859: Buddy Strings

Problem Restatement We are given two strings: s goal We need to return True if we can swap exactly two letters in s so that the result becomes equal to goal . A swap means choosing two different indices i and j , then exchanging: s[i] s[j] If no such swap exists, return False . Input and Output Item Meaning Input s , the original string Input goal , the...

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LeetCode 734: Sentence Similarity

Problem Restatement We are given two sentences represented as arrays of words: sentence1 sentence2 We are also given a list of similar word pairs: similarPairs Each pair: [x, y] means word x is similar to word y . Two sentences are similar if: They have the same number of words. Every word at index i in sentence1 is similar to the word at index i in sentence2 . A word...

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LeetCode 670: Maximum Swap

Problem Restatement We are given a non-negative integer num . We may swap two digits at most once. Return the maximum possible value we can get after doing zero or one swap. The official statement asks us to swap two digits at most once to get the maximum valued number. Input and Output Item Meaning Input A non-negative integer num Output The maximum integer possible after at most one digit...

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LeetCode 640: Solve the Equation

Problem Restatement We are given a linear equation as a string. The equation contains: Token Meaning x The variable integers Constant terms or coefficients + and - Operations = Separates left and right sides We need to solve for x . Return one of three possible answers: Case Return Exactly one solution "x=#value" No solution "No solution" Infinite solutions "Infinite solutions" If there is exactly one solution, the problem guarantees...

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LeetCode 639: Decode Ways II

Problem Restatement We are given a string s . The string contains digits and the wildcard character * . The normal decoding rule is: Code Letter "1" A "2" B ... ... "26" Z The wildcard * can represent any digit from "1" to "9" . It cannot represent "0" . We need to count how many valid ways the string can be decoded. Because the answer can be large,...

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LeetCode 758: Bold Words in String

Problem Restatement We are given: words s words is a list of strings. s is the target string. We need to wrap substrings of s with: ... if those substrings match any word in words . There are two important rules: If two bold parts overlap, merge them into one bold section. If two bold parts are adjacent, merge them too. Return the final formatted string. Input and...

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LeetCode 686: Repeated String Match

Problem Restatement We are given two strings a and b . We need to repeat string a some number of times so that b becomes a substring of the repeated string. Return the minimum number of repetitions needed. If it is impossible, return -1 . For example: a = "abcd" b = "cdabcdab" Repeating a two times gives: abcdabcd b is not a substring. Repeating a three times gives: abcdabcdabcd...

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LeetCode 636: Exclusive Time of Functions

Problem Restatement We are given n functions with IDs from 0 to n - 1 . We also receive a list of logs. Each log has this format: "{function_id}:{start_or_end}:{timestamp}" For example: "0:start:3" means function 0 starts at the beginning of timestamp 3 . "1:end:5" means function 1 ends at the end of timestamp 5 . The CPU is single-threaded, so only one function runs at a time. A function may...

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LeetCode 831: Masking Personal Information

Problem Restatement We are given a string s . The string represents either: Type Example Email address "LeetCode@LeetCode.com" Phone number "1(234)567-890" We need to return the masked version of the personal information. The masking rules are different for emails and phone numbers. An email contains the character "@" . A phone number contains digits and separator characters. Input and Output Item Meaning Input A personal information string s Output The...

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LeetCode 616: Add Bold Tag in String

Problem Restatement We are given a string s and an array of strings words . We need to wrap every substring of s that appears in words with a pair of bold tags: ... If two matching substrings overlap, they should be wrapped together with one pair of tags. If two matching substrings are adjacent, they should also be combined into one bold region. Return the final string after inserting...

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LeetCode 635: Design Log Storage System

Problem Restatement We need to design a log storage system. Each log has: Field Meaning id Unique log ID timestamp Time string in Year:Month:Day:Hour:Minute:Second format The timestamp format looks like this: "2017:01:01:23:59:59" All parts are zero-padded. The system needs two operations: Operation Meaning put(id, timestamp) Store a log retrieve(start, end, granularity) Return IDs whose timestamps are inside the inclusive range The granularity tells us how much of the timestamp to...

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LeetCode 981: Time Based Key-Value Store

Problem Restatement We need to design a data structure called TimeMap . It supports two operations: Operation Meaning set(key, value, timestamp) Store value for key at timestamp get(key, timestamp) Return the value for key with the largest stored timestamp <= timestamp If no stored timestamp is less than or equal to the query timestamp, return an empty string. The important guarantee is that, for each key, calls to set happen...

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LeetCode 830: Positions of Large Groups

Problem Restatement We are given a string s containing lowercase English letters. Consecutive equal characters form a group. For example: s = "abbxxxxzyy" The groups are: Group Interval "a" [0, 0] "bb" [1, 2] "xxxx" [3, 6] "z" [7, 7] "yy" [8, 9] A group is large if its length is at least 3 . We need to return the start and end indices of every large group, sorted by...

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LeetCode 664: Strange Printer

Problem Restatement We are given a string s . There is a strange printer with two rules: Rule Meaning Same character only In one turn, the printer can print only one repeated character Overwrite allowed A later turn can cover characters printed earlier The printer can choose any continuous segment of the string in one turn. Return the minimum number of turns needed to print the whole string. For example,...

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LeetCode 756: Pyramid Transition Matrix

Problem Restatement We are given a bottom row of blocks. Each block has a color represented by one letter. We are also given a list of allowed triples. A triple has length 3 . For example: "ABC" means: A B C If two adjacent blocks are A and B , then we may place C on top of them. Starting from the given bottom row, we need to decide whether...

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LeetCode 856: Score of Parentheses

Problem Restatement We are given a balanced parentheses string s . The score is defined by three rules: Form Score "()" 1 AB score(A) + score(B) (A) 2 * score(A) Here, A and B are balanced parentheses strings. We need to return the score of s . Input and Output Item Meaning Input A balanced parentheses string s Output The integer score of s Constraint 2 <= s.length <= 50...

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LeetCode 806: Number of Lines To Write String

Problem Restatement We are given a string s containing only lowercase English letters. We are also given an array widths of length 26 , where: widths[0] # width of 'a' widths[1] # width of 'b' widths[25] # width of 'z' Each line can contain at most 100 pixels. We write the characters of s from left to right. For each character, if it fits on the current line, we add...

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LeetCode 681: Next Closest Time

Problem Restatement We are given a valid time string in the format: HH:MM We need to form the next closest valid time by reusing only the digits that already appear in the input time. Each digit may be reused any number of times. For example, if the input is: time = "19:34" then the available digits are: 1, 9, 3, 4 We may use these digits repeatedly to form another...

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LeetCode 880: Decoded String at Index

Problem Restatement We are given an encoded string s and an integer k . The string is decoded from left to right using these rules: Character Meaning Letter Append the letter to the tape Digit d Repeat the entire current tape d - 1 more times Return the k th character of the final decoded string, using 1-based indexing. The decoded string can be extremely large, so we must not...

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LeetCode 730: Count Different Palindromic Subsequences

Problem Restatement We are given a string s . We need to count how many different non-empty palindromic subsequences appear in s . A subsequence is made by deleting zero or more characters without changing the order of the remaining characters. A palindrome reads the same forward and backward. The answer can be large, so we return it modulo: 10**9 + 7 The string contains only the characters 'a' ,...

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LeetCode 680: Valid Palindrome II

Problem Restatement We are given a string s . We need to return true if s can become a palindrome after deleting at most one character. Otherwise, return false . The string may also already be a palindrome, because deleting zero characters is allowed. The constraints say 1 <= s.length <= 10^5 , and s contains lowercase English letters. A palindrome reads the same from left to right and right...

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LeetCode 929: Unique Email Addresses

Problem Restatement We are given a list of email addresses. Each email has two parts: local-name@domain-name The problem defines two special rules that apply only to the local name. Rule 1: Ignore Dots In the local name: "." characters are ignored. For example: "alice.z" == "alicez" Rule 2: Ignore Everything After '+' If a plus sign appears in the local name: "+" then everything after it is ignored. For example:...

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LeetCode 955: Delete Columns to Make Sorted II

Problem Restatement We are given an array strs of strings. All strings have the same length. We may delete any set of column indices. When we delete a column, we delete that character position from every string. After deleting some columns, the remaining strings must be in lexicographic order: strs[0] <= strs[1] <= strs[2] <= ... <= strs[n - 1] Return the minimum number of deleted columns. The official constraints...

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LeetCode 524: Longest Word in Dictionary through Deleting

Problem Restatement We are given: s dictionary We may delete some characters from s . We need to return the longest string in dictionary that can be formed as a subsequence of s . If multiple answers have the same length, return the lexicographically smallest one. If no dictionary word can be formed, return the empty string. The official problem asks us to find the longest dictionary word obtainable by...

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LeetCode 804: Unique Morse Code Words

Problem Restatement We are given an array of lowercase English words. Each letter can be converted into Morse code. A word is transformed by replacing every letter with its Morse code and concatenating the results into one string. For example, the word "cab" becomes: "-.-." + ".-" + "-..." = "-.-..--..." We need to return how many different transformed strings appear among all words. The task is to count unique...

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LeetCode 854: K-Similar Strings

Problem Restatement We are given two strings s1 and s2 . The two strings are anagrams. That means they contain the same letters with the same frequencies. One operation lets us swap any two characters in s1 . We need to return the smallest number of swaps needed to turn s1 into s2 . This smallest number is called k . For example: s1 = "abc" s2 = "bca" One...

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LeetCode 828: Count Unique Characters of All Substrings of a Given String

Problem Restatement We are given a string s . For any string t , define countUniqueChars(t) as the number of characters that appear exactly once in t . We need to return the sum of countUniqueChars(t) over every substring t of s . The answer is guaranteed to fit in a 32-bit integer. Input and Output Item Meaning Input A string s Output Sum of unique-character counts over all substrings...

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LeetCode 953: Verifying an Alien Dictionary

Problem Restatement We are given: A list of words called words A string called order The string order describes the alphabet order used in an alien language. We need to check whether the words are sorted lexicographically according to this alien alphabet. Return true if the list is correctly sorted. Otherwise, return false . The official constraints are: Constraint Value 1 <= words.length <= 100 Number of words 1 <=...

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LeetCode 599: Minimum Index Sum of Two Lists

Problem Restatement We are given two string arrays, list1 and list2 . A common string is a string that appears in both arrays. For each common string, compute its index sum: index in list1 + index in list2 We need to return all common strings with the smallest index sum. The answer may be returned in any order. The problem guarantees that both lists have unique strings and that there...

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LeetCode 609: Find Duplicate File in System

Problem Restatement We are given a list of directory information strings. Each string has this format: "directory_path file1.txt(content1) file2.txt(content2) ..." For example: "root/a 1.txt(abcd) 2.txt(efgh)" means directory root/a contains: File Content 1.txt abcd 2.txt efgh We need to return groups of duplicate files. Two files are duplicates if they have exactly the same content, even if they have different names or live in different directories. Each returned group must contain...

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LeetCode 657: Robot Return to Origin

Problem Restatement A robot starts at position: (0, 0) We are given a string called moves . Each character tells the robot to move one step: Move Meaning U Move up D Move down L Move left R Move right After executing all moves, return True if the robot returns to the origin (0, 0) . Otherwise, return False . Input and Output Item Meaning Input A string moves Output...

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LeetCode 522: Longest Uncommon Subsequence II

Problem Restatement We are given an array of strings strs . We need to return the length of the longest uncommon subsequence among them. An uncommon subsequence is a string that is a subsequence of one string but not a subsequence of any other string in the array. If no such subsequence exists, return -1 . The official problem asks for the length of the longest uncommon subsequence among an...

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LeetCode 521: Longest Uncommon Subsequence I

Problem Restatement We are given two strings: a b We need to find the length of the longest uncommon subsequence between them. A subsequence is formed by deleting zero or more characters without changing the order of the remaining characters. An uncommon subsequence is a subsequence that appears in one string but not the other. If no uncommon subsequence exists, return: -1 The official problem asks for the longest uncommon...

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LeetCode 606: Construct String from Binary Tree

Problem Restatement We are given the root of a binary tree. We need to convert the tree into a string using preorder traversal: Step Meaning Visit current node Output node value Visit left subtree Wrap in parentheses Visit right subtree Wrap in parentheses However, unnecessary empty parentheses should be omitted. There is one important exception: If a node has a right child but no left child, we must include: ()...

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LeetCode 520: Detect Capital

Problem Restatement We are given a string word . We need to return true if the word uses capital letters correctly. A word uses capitals correctly if it matches one of these three patterns: Pattern Example All letters are uppercase "USA" All letters are lowercase "leetcode" Only the first letter is uppercase "Google" Otherwise, return false . The official problem asks us to return whether the usage of capitals in...

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LeetCode 544: Output Contest Matches

Problem Restatement We are given n teams labeled from 1 to n . Team 1 is the strongest team. Team n is the weakest team. In each round, we pair the strongest remaining team with the weakest remaining team, the second strongest with the second weakest, and so on. We need to return the final tournament bracket as a string. Pairs are written using parentheses and commas: (a,b) The input...

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LeetCode 516: Longest Palindromic Subsequence

Problem Restatement We are given a string s . We need to return the length of the longest subsequence of s that is also a palindrome. A subsequence is made by deleting zero or more characters without changing the order of the remaining characters. A palindrome reads the same forward and backward. The official problem asks for the length of the longest palindromic subsequence in s . The constraints are...

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LeetCode 567: Permutation in String

Problem Restatement We are given two strings s1 and s2 . Return true if s2 contains a permutation of s1 as a substring. Otherwise, return false . A permutation means the same characters with the same frequencies, but possibly in a different order. So the problem is asking: Does s2 contain any contiguous substring with exactly the same character counts as s1 ? The official constraints are 1 <= s1.length,...

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LeetCode 514: Freedom Trail

Problem Restatement We are given a circular ring represented by a string ring . We are also given a string key . The ring starts with ring[0] aligned at the 12:00 position. To spell each character in key , we may: Operation Cost Rotate the ring one position clockwise 1 Rotate the ring one position counterclockwise 1 Press the center button to select the current character 1 For each character...

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LeetCode 592: Fraction Addition and Subtraction

Problem Restatement We are given a string expression representing additions and subtractions of fractions. Each fraction has the form: numerator/denominator A fraction may have a leading + or - . We need to evaluate the whole expression and return the final result as an irreducible fraction string. If the result is an integer, we still return it with denominator 1 . For example: 2 must be returned as: "2/1" The...

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LeetCode 591: Tag Validator

Problem Restatement We are given a string called code . We must determine whether it is a valid code snippet according to several XML-like rules. The string may contain: Start tags: <TAG> End tags: </TAG> CDATA sections: <![CDATA[ ... ]]> Rules: Tag names must contain only uppercase English letters. Tag name length must be between 1 and 9 . Tags must be properly nested. The entire code must be wrapped...

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LeetCode 541: Reverse String II

Problem Restatement We are given a string s and an integer k . Starting from the beginning of the string, for every block of 2k characters, reverse the first k characters and leave the next k characters unchanged. There are two end cases: Remaining Characters Rule Fewer than k Reverse all remaining characters At least k but fewer than 2k Reverse the first k , leave the rest unchanged The...

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LeetCode 564: Find the Closest Palindrome

Problem Restatement We are given a string n representing an integer. Return the closest integer palindrome to n . The returned palindrome must not be n itself. If there are two palindromes with the same distance from n , return the smaller one. The input length is at most 18 , so the number can be too large for some fixed-width integer languages, but Python can handle it directly. The...

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LeetCode 678: Valid Parenthesis String

Problem Restatement We are given a string s . The string contains only three characters: Character Meaning '(' Opening parenthesis ')' Closing parenthesis '*' Wildcard character The wildcard '*' can be treated as one of three choices: Choice Meaning '(' Use it as an opening parenthesis ')' Use it as a closing parenthesis empty string Remove it We need to return true if there is some way to replace every...

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LeetCode 539: Minimum Time Difference

Problem Restatement We are given a list of time points in 24-hour "HH:MM" format. We need to return the minimum difference in minutes between any two time points. The clock wraps around after midnight, so the difference between "23:59" and "00:00" is 1 , not 1439 . The input contains at least two time points. Each time point is formatted as "HH:MM" . Input and Output Item Meaning Input A...

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LeetCode 604: Design Compressed String Iterator

Problem Restatement We need to design a class called StringIterator . The input is a compressed string. The compressed string has repeated groups of: letter + positive integer The integer tells us how many times that letter appears in the original uncompressed string. For example: "L1e2t1C1o1d1e1" represents: "LeetCode" because: Compressed Part Meaning L1 L appears once e2 e appears twice t1 t appears once C1 C appears once o1 o...

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LeetCode 588: Design In-Memory File System

Problem Restatement We need to design an in-memory file system. The file system must support four operations: Operation Behavior ls(path) If path is a file, return only that file name. If path is a directory, return its direct children in lexicographic order. mkdir(path) Create the directory path. If middle directories do not exist, create them too. addContentToFile(filePath, content) Create the file if missing, otherwise append content to the existing file....

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LeetCode 677: Map Sum Pairs

Problem Restatement We need to design a data structure called MapSum . It supports two operations: Operation Meaning insert(key, val) Store the value val for the string key sum(prefix) Return the total value of all keys that start with prefix If a key already exists, the new value replaces the old value. It does not add to the old value. For example: insert("apple", 3) sum("ap") # 3 insert("app", 2) sum("ap")...

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LeetCode 504: Base 7

Problem Restatement We are given an integer num . We need to return its representation in base 7 as a string. The official problem asks us to convert an integer from base 10 into base 7. Negative numbers should keep the negative sign. Input and Output Item Meaning Input An integer num Output A string Goal Return the base 7 representation of num Function shape: class Solution: def convertToBase7(self, num:...

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LeetCode 777: Swap Adjacent in LR String

Problem Restatement We are given two strings, start and result . Both strings contain only three characters: Character Meaning L A piece that can move left R A piece that can move right X An empty space We can make two kinds of moves: XL -> LX RX -> XR The first move lets L move one step left. The second move lets R move one step right. Return True...

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LeetCode 752: Open the Lock

Problem Restatement We have a lock with four wheels. Each wheel contains digits from 0 to 9 . The lock starts at: "0000" In one move, we can rotate exactly one wheel by one step. The digits wrap around: 9 -> 0 0 -> 9 Some lock states are deadends. If the lock reaches one of those states, it gets stuck and cannot move anymore. Given a list of deadends...

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LeetCode 727: Minimum Window Subsequence

Problem Restatement We are given two strings, s1 and s2 . We need to return the shortest contiguous substring of s1 such that s2 is a subsequence of that substring. A subsequence means the characters appear in the same order, but they do not need to be adjacent. If multiple shortest substrings exist, return the one with the left-most starting index. If no valid substring exists, return an empty string....

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LeetCode 926: Flip String to Monotone Increasing

Problem Restatement We are given a binary string s . Each character is either '0' or '1' . A binary string is monotone increasing when all 0 s come before all 1 s. It may contain only 0 s, only 1 s, or both. Valid examples: "000" "111" "000111" "001111" Invalid examples: "10" "101" "010" We may flip any character: 0 -> 1 1 -> 0 Return the minimum number...

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LeetCode 751: IP to CIDR

Problem Restatement We are given a starting IPv4 address ip and an integer n . We need to cover exactly n consecutive IP addresses starting from ip . The answer should be a list of CIDR blocks with the smallest possible length. A CIDR block looks like this: "123.45.67.89/20" The number after / is the prefix length. A larger prefix length means a smaller block. For example, /32 covers exactly...

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LeetCode 726: Number of Atoms

Problem Restatement We are given a valid chemical formula string. The formula contains atom names, numbers, and parentheses. We need to count how many times each atom appears, then return the result as a string sorted by atom name. An atom name starts with one uppercase letter. It may be followed by lowercase letters. Examples: "H" "Mg" "Ca" A number after an atom means the atom appears that many times....

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LeetCode 676: Implement Magic Dictionary

Problem Restatement We need to design a data structure called MagicDictionary . It supports two operations: Operation Meaning buildDict(dictionary) Store a list of distinct words search(searchWord) Return whether searchWord can become a dictionary word by changing exactly one character The important rule is exactly one character must be changed. Zero changes do not count. Two or more changes do not count. The modified word must exist in the dictionary. For...

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LeetCode 537: Complex Number Multiplication

Problem Restatement We are given two strings representing complex numbers. The format is: "real+imaginaryi" For example: "1+1i" represents: 1 + i We need to multiply the two complex numbers and return the result in the same string format. The imaginary unit satisfies: i^2 = -1 Input and Output Item Meaning Input Two strings representing complex numbers Output Their product as a complex-number string Format "a+bi" Values Integers, possibly negative Function...

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LeetCode 536: Construct Binary Tree from String

Problem Restatement We are given a string representing a binary tree. The string format is: node(left_subtree)(right_subtree) Each node starts with an integer value. If a node has children, the left subtree appears first inside parentheses. The right subtree appears second inside another pair of parentheses. We need to reconstruct the binary tree and return its root. The string may contain negative integers. Input and Output Item Meaning Input A string...

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LeetCode 535: Encode and Decode TinyURL

Problem Restatement We need to design a class that can shorten a long URL and later recover the original URL. The class has two methods: encode(longUrl) and: decode(shortUrl) encode receives a valid long URL and returns a tiny URL. decode receives a tiny URL generated by the same object and returns the original long URL. The problem does not restrict the algorithm. It only requires that a URL encoded by...

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LeetCode 583: Delete Operation for Two Strings

Problem Restatement We are given two strings, word1 and word2 . In one step, we may delete exactly one character from either string. We need to return the minimum number of deletion steps required to make the two strings equal. The strings contain only lowercase English letters, and each length is between 1 and 500 . Input and Output Item Meaning word1 First lowercase English string word2 Second lowercase English...

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LeetCode 557: Reverse Words in a String III

Problem Restatement We are given a string s . The string contains words separated by single spaces. We need to reverse the characters inside each word while keeping: The word order unchanged. The spaces in the same positions. The problem guarantees there are no leading or trailing spaces, and words are separated by exactly one space. Input and Output Item Meaning Input A string s Output A string with each...

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LeetCode 556: Next Greater Element III

Problem Restatement We are given a positive integer n . We need to rearrange its digits to form the smallest integer that is greater than n . The new integer must use exactly the same digits as n . If no greater integer can be formed, return -1 . If the answer is greater than the 32-bit signed integer limit, return -1 . The official constraint is: 1 <= n...

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LeetCode 555: Split Concatenated Strings

Problem Restatement We are given an array of strings strs . We can concatenate the strings into a loop. For each string, we may use it normally or reversed. The strings must stay in the same order as given. After forming the loop, we choose one cut point. Cutting the loop turns it into a normal string. Our goal is to return the lexicographically largest possible string. The input strings...

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LeetCode 553: Optimal Division

Problem Restatement We are given an integer array nums . The numbers are used in order, with division between adjacent numbers. For example, if: nums = [2, 3, 4] then the default expression is: "2/3/4" Division is evaluated from left to right unless parentheses change the order. We may add parentheses anywhere to change the evaluation order. Our goal is to return the expression with the maximum possible value. The...

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LeetCode 552: Student Attendance Record II

Problem Restatement We are given an integer n . We need to count how many attendance records of length n are eligible for an award. Each record contains only three characters: Character Meaning "A" Absent "L" Late "P" Present A record is valid only if: It has strictly fewer than 2 absences. It never has 3 or more consecutive late days. Return the number of valid records modulo 10^9 +...

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LeetCode 527: Word Abbreviation

Problem Restatement We are given an array of distinct lowercase words. For each word, we need to return its minimal unique abbreviation, while keeping the output in the same order as the input. The abbreviation starts with a prefix, then the number of abbreviated middle characters, then the last character. If the abbreviation is not shorter than the original word, we keep the original word. For example: "internal" -> "i6l"...

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LeetCode 551: Student Attendance Record I

Problem Restatement We are given a string s representing one student's attendance record. Each character has one meaning: Character Meaning "A" Absent "L" Late "P" Present The student gets an attendance award only if both rules are satisfied: The student has strictly fewer than 2 absences in total. The student is never late for 3 or more consecutive days. Return true if the student can receive the award. Otherwise, return...

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LeetCode 425: Word Squares

Problem Restatement We are given an array of unique strings: words All words have the same length. Return all word squares that can be built from these words. The same word may be used multiple times. A word square is a list of words where the k th row and the k th column read the same string. The answer may be returned in any order. The source statement says...

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LeetCode 424: Longest Repeating Character Replacement

Problem Restatement We are given a string s and an integer k . The string contains only uppercase English letters. In one operation, we may choose any character and change it to any other uppercase English letter. We may perform at most k operations. Return the length of the longest substring that can be made of one repeated letter after at most k replacements. The official examples include s =...

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LeetCode 474: Ones and Zeroes

Problem Restatement We are given an array of binary strings: strs We are also given two integers: m n We can choose some strings from strs . The chosen strings must use at most: m zeros n ones Each string can be chosen at most once. We need to return the largest possible number of strings we can choose. The official problem asks for the size of the largest subset...

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LeetCode 423: Reconstruct Original Digits from English

Problem Restatement We are given a string s . The string contains shuffled letters from English digit words: "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" The letters are out of order. We must reconstruct the original digits and return them in ascending order. The input is guaranteed to be valid. The constraints allow s.length up to 10^5 . The source examples include "owoztneoer" -> "012" and "fviefuro"...

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LeetCode 500: Keyboard Row

Problem Restatement We are given an array of strings words . Return the words that can be typed using letters from only one row of an American keyboard. The keyboard rows are: Row Letters First row qwertyuiop Second row asdfghjkl Third row zxcvbnm Letter case does not matter. For example, A and a belong to the same row. The word must use letters from only one row. The original word...

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LeetCode 472: Concatenated Words

Problem Restatement We are given an array of unique strings: words We need to return all words in the array that can be formed by concatenating at least two shorter words from the same array. The shorter words do not need to be distinct. The same word may be used more than once. The official statement defines a concatenated word as a word comprised entirely of at least two shorter...

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LeetCode 471: Encode String with Shortest Length

Problem Restatement We are given a non-empty lowercase string s . We need to encode it so that the encoded string has the shortest possible length. The encoding rule is: k[encoded_string] where encoded_string is repeated exactly k times. If encoding does not make the string shorter, we should keep the original string. If there are multiple shortest answers, returning any one of them is allowed. The input length is at...

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LeetCode 422: Valid Word Square

Problem Restatement We are given an array of strings: words Return True if the strings form a valid word square. A word square means: row k == column k for every valid index k . The words may have different lengths, so we must be careful with missing characters. The source statement defines a valid word square as a sequence where the k th row and k th column read...

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LeetCode 420: Strong Password Checker

Problem Restatement We are given a string password . Return the minimum number of steps needed to make it strong. A strong password must satisfy all three rules: Rule Requirement Length At least 6 characters and at most 20 characters Character types At least one lowercase letter, one uppercase letter, and one digit Repetition No three same characters in a row In one step, we may: Insert one character. Delete...

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LeetCode 468: Validate IP Address

Problem Restatement We are given a string queryIP . We need to return one of three strings: "IPv4" "IPv6" "Neither" Return "IPv4" if the string is a valid IPv4 address. Return "IPv6" if the string is a valid IPv6 address. Otherwise, return "Neither" . A valid IPv4 address has four decimal parts separated by dots. Each part must be between 0 and 255 , and leading zeros are not allowed....

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LeetCode 418: Sentence Screen Fitting

Problem Restatement We are given: Value Meaning sentence A list of words rows Number of screen rows cols Number of columns per row We want to place the sentence repeatedly on the screen. Rules: Words must appear in the original order. A word cannot be split across lines. Consecutive words are separated by one space. The sentence repeats cyclically. Return how many complete times the sentence fits on the screen....

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LeetCode 395: Longest Substring with At Least K Repeating Characters

Problem Restatement We are given a string s and an integer k . We need to return the length of the longest substring where every character in that substring appears at least k times. A substring must be contiguous. If no valid substring exists, return 0 . The string contains only lowercase English letters. The length of s is at most 10^4 , and k can be larger than the...

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LeetCode 320: Generalized Abbreviation

Problem Restatement We are given a lowercase string word . Return all possible generalized abbreviations of word . A generalized abbreviation replaces any number of non-overlapping and non-adjacent substrings with their lengths. For example, "abcde" can become "a3e" , "1bcd1" , "5" , or "abcde" . The input length is at most 15 , and the answer can be returned in any order. Input and Output Item Meaning Input A...

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LeetCode 467: Unique Substrings in Wraparound String

Problem Restatement We are given a string s . There is also an infinite wraparound base string: "...zabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcd..." We need to count how many different non-empty substrings of s also appear in this infinite base string. The official problem asks for the number of unique non-empty substrings of s that are present in base . A substring appears in the base string if its characters are consecutive in alphabet order,...

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LeetCode 299: Bulls and Cows

Problem Restatement We are given two equal-length digit strings: secret guess We need to return a hint in this format: "xAyB" where: Symbol Meaning x Number of bulls y Number of cows A bull is a digit that has the same value and the same position in both strings. A cow is a digit that exists in both strings but is in the wrong position. Duplicate digits must be counted...

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LeetCode 394: Decode String

Problem Restatement We are given an encoded string s . The encoding rule is: k[encoded_string] This means encoded_string should be repeated exactly k times. For example: "3[a]" -> "aaa" "2[bc]" -> "bcbc" The encoding may be nested: "3[a2[c]]" -> "accaccacc" The input is guaranteed to be valid. Brackets are well-formed, repeat counts are positive integers, and digits only appear as repeat counts. The decoded output length will not exceed 10^5...

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LeetCode 318: Maximum Product of Word Lengths

Problem Restatement We are given an array of lowercase strings words . Return the maximum value of: len(words[i]) * len(words[j]) where the two words do not share any common letters. If no such pair exists, return 0 . The official constraints include 2 <= words.length <= 1000 , 1 <= words[i].length <= 1000 , and each word contains only lowercase English letters. Input and Output Item Meaning Input A list...

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LeetCode 466: Count The Repetitions

Problem Restatement We are given two strings and two repetition counts: s1, n1 s2, n2 The notation [s, n] means string s repeated n times. For example: ["abc", 3] = "abcabcabc" Define: str1 = [s1, n1] str2 = [s2, n2] We need to find the maximum integer m such that: [str2, m] can be obtained from str1 by deleting some characters. In other words, we need to count how many...

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LeetCode 297: Serialize and Deserialize Binary Tree

Problem Restatement We need to design a codec for a binary tree. The codec has two operations: serialize(root) deserialize(data) serialize(root) converts a binary tree into a string. deserialize(data) converts that string back into the original binary tree. There is no required string format. The only requirement is that a tree serialized by our method can be deserialized back into the same tree structure. The source statement also clarifies that we...

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LeetCode 443: String Compression

Problem Restatement We are given an array of characters: chars We must compress the array in-place using these rules: Case Output Single character Keep the character only Repeated character Write character followed by count For example: ["a","a","b","b","c","c","c"] becomes: ["a","2","b","2","c","3"] The compressed result must be written back into the same array. The function returns the new length after compression. Counts larger than 9 must be split into multiple characters. For example:...

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LeetCode 392: Is Subsequence

Problem Restatement We are given two strings: s t We need to return True if s is a subsequence of t . A subsequence is formed by deleting zero or more characters from a string without changing the order of the remaining characters. For example: s = "abc" t = "ahbgdc" The characters a , b , and c appear in t in the same order. So s is a...

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LeetCode 415: Add Strings

Problem Restatement We are given two non-negative integers as strings: num1 num2 Return their sum as a string. We cannot use a built-in big integer library. We also cannot directly convert the whole input strings into integers. So this is manual addition, the same way we add numbers on paper. The source problem states that num1 and num2 contain only digits, have no leading zeroes except "0" itself, and each...

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LeetCode 224: Basic Calculator

Problem Restatement We are given a string s representing a valid arithmetic expression. The expression may contain: Token Meaning Digits Non-negative integers + Addition - Subtraction or unary negative ( Start of a sub-expression ) End of a sub-expression Space Ignored We need to evaluate the expression and return the integer result. We cannot use built-in expression evaluators such as eval . The official statement asks us to implement a...

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LeetCode 345: Reverse Vowels of a String

Problem Restatement We are given a string s . Reverse only the vowels in the string and return the resulting string. The vowels are: a, e, i, o, u A, E, I, O, U All non-vowel characters must stay in their original positions. For example: "hello" becomes: "holle" because the vowels: e, o are reversed to: o, e Input and Output Item Meaning Input String s Output String with reversed...

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LeetCode 316: Remove Duplicate Letters

Problem Restatement We are given a string s containing lowercase English letters. We need to remove duplicate letters so that every distinct letter appears exactly once. Among all valid results, return the lexicographically smallest one. The official constraints are: Constraint Value s.length 1 <= s.length <= 10^4 Characters Lowercase English letters This problem is also the same as LeetCode 1081, Smallest Subsequence of Distinct Characters. ( github.com ) Input and...

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LeetCode 344: Reverse String

Problem Restatement We are given an array of characters s . Reverse the array in place. This means we must modify the original array directly and use only O(1) extra memory. The function should not return a new array. It should change s itself. For example: ["h", "e", "l", "l", "o"] should become: ["o", "l", "l", "e", "h"] Input and Output Item Meaning Input Array of characters s Output Nothing...

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LeetCode 294: Flip Game II

Problem Restatement We are given a string currentState containing only: "+" "-" Two players take turns. On each turn, a player chooses two consecutive plus signs: "++" and flips them into: "--" The player who cannot make a move loses. We start first. We need to return whether the starting player can guarantee a win. The source statement defines the game this way: players take turns flipping two consecutive "++"...

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LeetCode 293: Flip Game

Problem Restatement We are given a string currentState containing only two characters: "+" "-" A valid move means choosing two consecutive plus signs: "++" and flipping them into: "--" We need to return all possible states of the string after making exactly one valid move. The answer may be returned in any order. If there is no valid move, return an empty list. The source statement defines the game this...

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LeetCode 273: Integer to English Words

Problem Restatement We are given a non-negative integer num . Return its English words representation. Examples: 123 -> "One Hundred Twenty Three" 12345 -> "Twelve Thousand Three Hundred Forty Five" 1234567 -> "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven" 0 -> "Zero" The constraint is: 0 <= num <= 2^31 - 1 So the largest possible number is less than 2.15 billion. We only need scale...

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LeetCode 291: Word Pattern II

Problem Restatement We are given: pattern s We need to check whether s follows pattern . Each character in pattern must map to a non-empty substring of s . The mapping must be bijective: Rule Meaning One character maps to one substring The same pattern character must always use the same substring One substring maps to one character Two different pattern characters cannot use the same substring Full match All...

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LeetCode 290: Word Pattern

Problem Restatement We are given two strings: pattern s pattern contains letters. s contains words separated by spaces. We need to check whether s follows the same pattern. “Follows the same pattern” means there must be a bijection between pattern letters and words. In other words, each pattern letter maps to exactly one word, and each word maps back to exactly one pattern letter. For example: pattern = "abba" s...

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LeetCode 271: Encode and Decode Strings

Problem Restatement We need to design two methods: encode(strs) decode(s) The encode method receives a list of strings and converts it into one string. The decode method receives that encoded string and reconstructs the original list. We cannot use serialization helpers such as eval . The strings may contain any of the 256 valid ASCII characters, including delimiters, digits, spaces, and empty strings. The constraints are 1 <= strs.length <=...

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LeetCode 242: Valid Anagram

Problem Restatement We are given two strings: s and t We need to determine whether t is an anagram of s . Two strings are anagrams if: They contain exactly the same characters Every character appears the same number of times The order does not matter. LeetCode examples include: Input: s = "anagram", t = "nagaram" Output: true and: Input: s = "rat", t = "car" Output: false The problem...

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LeetCode 269: Alien Dictionary

Problem Restatement We are given a list of words sorted according to an unknown alien alphabet. We need to infer one possible order of the alien alphabet. Return a string containing all unique letters in a valid order. If no valid order exists, return: "" If multiple valid orders exist, any one of them is accepted. The constraints are 1 <= words.length <= 100 , 1 <= words[i].length <= 100...

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LeetCode 288: Unique Word Abbreviation

Problem Restatement We are given a dictionary of words. We need to design a class that can answer this query: isUnique(word) It returns whether word has a unique abbreviation in the dictionary. The abbreviation rule is: If the word length is less than 3 , the abbreviation is the word itself. Otherwise: first letter + number of middle letters + last letter For example: "dog" -> "d1g" "internationalization" -> "i18n"...

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LeetCode 267: Palindrome Permutation II

Problem Restatement We are given a string s . Return all palindromic permutations of s without duplicates. A palindrome reads the same forward and backward. If no palindromic permutation exists, return an empty list. The answer may be returned in any order. The constraints are 1 <= s.length <= 16 , and s contains only lowercase English letters. Input and Output Item Meaning Input A lowercase string s Output List...

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LeetCode 214: Shortest Palindrome

Problem Restatement We are given a string s . We may add characters only in front of s . We need to return the shortest palindrome that can be created this way. The official statement says we can convert s to a palindrome by adding characters in front of it, and we must return the shortest palindrome possible. Input and Output Item Meaning Input String s Output Shortest palindrome formed...

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LeetCode 266: Palindrome Permutation

Problem Restatement We are given a string s . Return True if some permutation of s can form a palindrome. Return False otherwise. A palindrome reads the same forward and backward. For example: "aba" "racecar" "abba" The order of characters in s can be rearranged. We only need to know whether at least one palindrome arrangement exists. The constraints are 1 <= s.length <= 5000 , and s contains only...

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LeetCode 125: Valid Palindrome

Problem Restatement We are given a string s . We need to decide whether it is a palindrome after: Converting uppercase letters to lowercase. Removing all non-alphanumeric characters. Alphanumeric characters are letters and digits. The official problem asks us to return true if the cleaned string reads the same forward and backward. For example: s = "A man, a plan, a canal: Panama" After cleaning, it becomes: "amanaplanacanalpanama" This reads...

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LeetCode 97: Interleaving String

Problem Restatement We are given three strings: s1: str s2: str s3: str We need to decide whether s3 can be formed by interleaving s1 and s2 . Interleaving means we take all characters from s1 and all characters from s2 , while preserving the original order inside each string. The characters may be mixed together. For example: s1 = "abc" s2 = "def" A valid interleaving is: "adbcef" because...

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LeetCode 171: Excel Sheet Column Number

Problem Restatement We are given an Excel column title: columnTitle We need to return its corresponding column number. Excel columns are labeled like this: Title Number "A" 1 "B" 2 "Z" 26 "AA" 27 "AB" 28 "ZY" 701 This problem is the reverse of LeetCode 168. The input contains only uppercase English letters. The constraints guarantee the answer fits in a 32-bit integer. Input and Output Item Meaning Input Excel...

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LeetCode 72: Edit Distance

Problem Restatement We are given two strings, word1 and word2 . We need to return the minimum number of operations required to convert word1 into word2 . There are three allowed operations: Insert one character Delete one character Replace one character The official constraints are 0 <= word1.length, word2.length <= 500 , and both strings contain lowercase English letters. Input and Output Item Meaning Input Two strings word1 and word2...

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LeetCode 93: Restore IP Addresses

Problem Restatement We are given a string s containing only digits. We need to return all possible valid IP addresses that can be formed by inserting dots into s . We cannot reorder digits. We cannot remove digits. We only insert exactly three dots so that the string becomes four valid IP address segments. A valid IP address has exactly four integers separated by dots. Each integer must be between...

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LeetCode 71: Simplify Path

Problem Restatement We are given a Unix-style absolute file path. We need to simplify it into its canonical form. The rules are: A single dot "." means the current directory. A double dot ".." means move to the parent directory. Multiple consecutive slashes should be treated as a single slash. Any other name should be treated as a normal directory name. The result must: Start with a single slash Have...

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LeetCode 91: Decode Ways

Problem Restatement We are given a string s containing only digits. The digits encode letters using this mapping: "A" -> "1" "B" -> "2" ... "Z" -> "26" We need to count how many different ways the string can be decoded. A digit group is valid only if it maps to a number from 1 to 26 . So: "12" can be decoded as: "1" "2" -> "AB" "12" ->...

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LeetCode 168: Excel Sheet Column Title

Problem Restatement We are given a positive integer: columnNumber We need to return the corresponding Excel column title. Excel columns are labeled like this: Number Title 1 "A" 2 "B" 26 "Z" 27 "AA" 28 "AB" 701 "ZY" The problem asks us to convert an integer into this Excel-style column label. The constraint is 1 <= columnNumber <= 2^31 - 1 . Input and Output Item Meaning Input Positive integer...

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LeetCode 115: Distinct Subsequences

Problem Restatement We are given two strings: s t We need to count how many distinct subsequences of s equal t . A subsequence is formed by deleting zero or more characters without changing the order of the remaining characters. The official problem asks for the number of distinct subsequences of s equal to t . For example: s = "rabbbit" t = "rabbit" The answer is: 3 because there...

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LeetCode 68: Text Justification

Problem Restatement We are given a list of words and an integer maxWidth . We need to format the words into lines of exactly maxWidth characters. The rules are: Pack as many words as possible into each line. Each line must have exactly maxWidth characters. For normal lines, spaces should be distributed as evenly as possible between words. If spaces cannot divide evenly, the left gaps get more spaces than...

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LeetCode 166: Fraction to Recurring Decimal

Problem Restatement We are given two integers: numerator denominator They represent the fraction: numerator / denominator We need to return the decimal representation as a string. If the fractional part repeats forever, we put the repeating part inside parentheses. For example: 1 / 2 = "0.5" 2 / 1 = "2" 2 / 3 = "0.(6)" 4 / 333 = "0.(012)" The denominator is never zero, and the result string...

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LeetCode 67: Add Binary

Problem Restatement We are given two binary strings, a and b . Each string contains only characters "0" and "1" . We need to return their sum as a binary string. The official constraints are 1 <= a.length, b.length <= 10^4 , both strings contain only binary digits, and neither string has leading zeroes except the string "0" itself. Input and Output Item Meaning Input Two binary strings a and...

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LeetCode 140: Word Break II

Problem Restatement We are given: Name Meaning s A string without spaces wordDict A list of valid dictionary words We need to insert spaces into s so that every piece is a valid dictionary word. Return all possible valid sentences. The same dictionary word may be reused multiple times. The answer may be returned in any order. LeetCode gives constraints including 1 <= s.length <= 20 , 1 <= wordDict.length...

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LeetCode 139: Word Break

Problem Restatement We are given: Name Meaning s A string we want to split wordDict A list of valid dictionary words Return True if s can be segmented into a sequence of one or more dictionary words. The same dictionary word may be used multiple times. If no complete segmentation exists, return False . LeetCode states the task as checking whether s can be segmented into a space-separated sequence of...

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LeetCode 65: Valid Number

Problem Restatement We are given a string s . We need to return true if s represents a valid number. Otherwise, return false . A valid number can have: A decimal number or an integer Optionally, an exponent part using e or E , followed by an integer The official examples include valid strings such as: "2" "0089" "-0.1" "+3.14" "4." "-.9" "2e10" "-90E3" "3e+7" "+6e-1" "53.5e93" "-123.456e789" And invalid...

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LeetCode 165: Compare Version Numbers

Problem Restatement We are given two version strings: version1 version2 Each version string contains revisions separated by dots. For example: "1.2.10" has revisions: 1, 2, 10 We need to compare the two versions revision by revision from left to right. Return: Return value Meaning -1 version1 is smaller 1 version1 is larger 0 Both versions are equal Leading zeros are ignored, so: "1.01" "1.001" are equal. If one version has...

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LeetCode 412: Fizz Buzz

Problem Restatement We are given an integer n . Return a string array answer of length n . For every integer i from 1 to n : Condition Output i is divisible by both 3 and 5 "FizzBuzz" i is divisible by 3 "Fizz" i is divisible by 5 "Buzz" None of the above i as a string The array is 1-indexed in the problem description, meaning answer[i] describes the...

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LeetCode 87: Scramble String

Problem Restatement We are given two strings: s1: str s2: str Both strings have the same length. We need to return whether s2 can be produced by scrambling s1 . A scramble is formed recursively: If the string length is 1 , stop. Otherwise, split the string into two non-empty parts. Either keep the two parts in the same order or swap them. Apply the same process recursively to each...

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LeetCode 389: Find the Difference

Problem Restatement We are given two strings s and t . String t is created by shuffling s and adding one extra letter at a random position. We need to return the added letter. For example: s = "abcd" t = "abcde" The added letter is: "e" The strings contain only lowercase English letters. Also, t always has exactly one more character than s . Input and Output Item Meaning...

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LeetCode 439: Ternary Expression Parser

Problem Restatement We are given a valid string expression representing nested ternary expressions. A ternary expression has the form: condition ? true_value : false_value The condition is always either: T F where T means true and F means false. The expression contains only: Character Meaning T true F false 0 to 9 single digit value ? starts a ternary choice : separates true branch and false branch Each number has...

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LeetCode 411: Minimum Unique Word Abbreviation

Problem Restatement We are given a target string and a dictionary of strings. We need to find the shortest abbreviation of target that does not conflict with any abbreviation of any word in the dictionary. An abbreviation may replace substrings with their lengths. Each number or letter in the abbreviation counts as length 1 . For example, "a32bc" has abbreviation length 4 . If multiple shortest answers exist, any one...

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LeetCode 488: Zuma Game

Problem Restatement We are given a row of colored balls called board and some balls in our hand called hand . Each ball has one of five colors: Character Color R Red Y Yellow B Blue G Green W White On each move, we choose one ball from hand and insert it anywhere in board . After insertion, if there are three or more consecutive balls of the same color,...

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LeetCode 388: Longest Absolute File Path

Problem Restatement We are given a string input that represents a file system. Each file or directory appears on its own line. A newline character means a new entry: "\n" A tab character at the start of a line tells us the depth: "\t" A file is identified by having a dot . in its name. We need to return the length of the longest absolute path to any file....

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LeetCode 438: Find All Anagrams in a String

Problem Restatement We are given two strings: Name Meaning s The string we search inside p The pattern string We need to return all starting indices in s where an anagram of p begins. An anagram uses the same characters with the same frequencies, but the order can be different. For example: "abc", "bac", "cba" are all anagrams of each other. The official problem asks us to return all start...

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LeetCode 38: Count and Say

Problem Restatement We are given a positive integer n . We need to return the n th term of the count-and-say sequence. The sequence starts with: countAndSay(1) = "1" For every n > 1 , the next term is built by reading the previous term using run-length encoding. Run-length encoding means grouping consecutive equal digits, then writing: count + digit For example: "111221" has groups: "111", "22", "1" So it...

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LeetCode 387: First Unique Character in a String

Problem Restatement We are given a string s . We need to find the first character that appears exactly once in the string and return its index. If every character appears more than once, return -1 . The string contains only lowercase English letters, and its length can be up to 10^5 . Input and Output Item Meaning Input A string s Output Index of the first non-repeating character Return...

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LeetCode 187: Repeated DNA Sequences

Problem Restatement We are given a string s representing a DNA sequence. A DNA sequence contains only these four characters: Character Meaning A Adenine C Cytosine G Guanine T Thymine We need to find all 10-letter-long substrings that appear more than once in s . The answer can be returned in any order. The official statement asks for all 10-letter-long sequences that occur more than once in a DNA molecule....

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LeetCode 409: Longest Palindrome

Problem Restatement We are given a string s . The string contains lowercase and uppercase English letters. We may rearrange the letters in any order. Return the length of the longest palindrome that can be built using those letters. Letters are case-sensitive, so: "A" != "a" The problem asks for the length only, not the palindrome itself. The constraints are 1 <= s.length <= 2000 , and the string contains...

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LeetCode 340: Longest Substring with At Most K Distinct Characters

Problem Restatement We are given a string s and an integer k . Return the length of the longest substring of s that contains at most k distinct characters. A substring must be contiguous. For example: s = "eceba" k = 2 The substring "ece" contains only two distinct characters: e, c Its length is 3 , so the answer is 3 . Input and Output Item Meaning Input A...

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LeetCode 282: Expression Add Operators

Problem Restatement We are given: A string num containing only digits. An integer target . We need to insert the operators: +, -, * between the digits so that the final mathematical expression evaluates exactly to target . We must return all valid expressions. Digits must remain in the original order. We may also choose to join consecutive digits into a larger number. For example: "123" can become: "1+2+3" "12-3"...

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LeetCode 459: Repeated Substring Pattern

Problem Restatement We are given a string s . We need to check whether s can be constructed by taking a non-empty substring and repeating it multiple times. The substring must be shorter than the whole string, because repeating the entire string once does not count. The official problem asks whether s can be built by appending multiple copies of a substring together. For example: s = "abab" This can...

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LeetCode 208: Implement Trie Prefix Tree

Problem Restatement We need to implement a Trie, also called a prefix tree. The Trie must support three operations: Operation Meaning insert(word) Add word into the Trie search(word) Return True if the exact word exists startsWith(prefix) Return True if any inserted word starts with prefix The official problem asks us to implement the Trie class with insert , search , and startsWith . The Trie stores lowercase English words and...

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LeetCode 186: Reverse Words in a String II

Problem Restatement We are given a character array s . The array contains words separated by single spaces. There are no leading spaces, no trailing spaces, and each pair of words is separated by exactly one space. We need to reverse the order of the words in-place. The internal order of characters inside each word should stay the same. The problem requires solving it without allocating extra space. Input and...

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LeetCode 408: Valid Word Abbreviation

Problem Restatement We are given a word and an abbreviation. The abbreviation may contain lowercase letters and digits. A digit sequence means we skip that many characters in the original word. We must return whether the abbreviation correctly represents the word. Numbers cannot have leading zeroes. For example, "01" is invalid. The official problem describes abbreviations as replacing non-adjacent, non-empty substrings with their lengths. It gives examples such as "s10n"...

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LeetCode 161: One Edit Distance

Problem Restatement We are given two strings: s t Return True if they are exactly one edit distance apart. An edit means one of these operations: Operation Meaning Insert Insert exactly one character into s to get t Delete Delete exactly one character from s to get t Replace Replace exactly one character in s to get t The key word is exactly. If the strings are already equal, the...

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LeetCode 434: Number of Segments in a String

Problem Restatement We are given a string s . A segment is a contiguous sequence of non-space characters. We need to count how many segments appear in the string. For example: s = "Hello, my name is John" contains these segments: "Hello," "my" "name" "is" "John" So the answer is: 5 The official problem defines a segment as a contiguous sequence of non-space characters. The string may contain leading spaces,...

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LeetCode 484: Find Permutation

Problem Restatement We are given a string s made only of two characters: Character Meaning I The next number must be greater D The next number must be smaller We need to return the lexicographically smallest permutation of numbers from 1 to n , where: n = len(s) + 1 The permutation must match the pattern in s . For each index i : Pattern Required relation s[i] == "I"...

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LeetCode 58: Length of Last Word

Problem Restatement We are given a string s containing English letters and spaces. We need to return the length of the last word in the string. A word is a maximal substring made only of non-space characters. The string contains at least one word. The official constraints are 1 <= s.length <= 10^4 , and s contains only English letters and spaces. Input and Output Item Meaning Input A string...

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LeetCode 385: Mini Parser

Problem Restatement We are given a string s . The string represents either: A single integer A nested list of integers Examples: "324" "[123,[456,[789]]]" "[-1,[2,-3],4]" We need to parse this string and return a NestedInteger object. The NestedInteger class is already provided by LeetCode. We only need to implement: deserialize(s) Input and Output Item Meaning Input A serialized string s Output A NestedInteger object Integer format May be positive or...

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LeetCode 358: Rearrange String k Distance Apart

Problem Restatement We are given a non-empty lowercase string s and an integer k . We need to rearrange the characters of s so that the same characters are at least distance k from each other. If this is possible, return any valid rearranged string. If this is impossible, return an empty string "" . For example, if character a appears at index i , then the next a can...

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LeetCode 336: Palindrome Pairs

Problem Restatement We are given a 0-indexed array of unique strings words . Return all pairs of different indices (i, j) such that: words[i] + words[j] is a palindrome. A palindrome reads the same forward and backward. For example: "abccba" "racecar" "" are palindromes. The problem asks for all valid pairs, not only one pair. The words are unique, and LeetCode asks for an algorithm close to the total input...

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LeetCode 132: Palindrome Partitioning II

Problem Restatement We are given a string s . We need to cut the string into substrings so that every substring is a palindrome. Return the minimum number of cuts needed. LeetCode states the problem as partitioning s so every substring is a palindrome, then returning the minimum cuts needed for such a palindrome partitioning. The constraints are 1 <= s.length <= 2000 , and s contains lowercase English letters...

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LeetCode 433: Minimum Genetic Mutation

Problem Restatement We are given two gene strings: Name Meaning startGene The starting gene endGene The target gene Each gene string has length 8 and contains only these characters: "A", "C", "G", "T" One mutation means changing exactly one character. For example: AACCGGTT -> AACCGGTA This is one mutation because only the last character changed. We are also given a gene bank. A gene is valid only if it appears...

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LeetCode 159: Longest Substring with At Most Two Distinct Characters

Problem Restatement We are given a string s . We need to return the length of the longest substring that contains at most two distinct characters. A substring must be contiguous. For example: s = "eceba" The substring: "ece" contains only two distinct characters: "e", "c" Its length is: 3 So the answer is: 3 LeetCode gives examples such as s = "eceba" with output 3 , and s =...

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LeetCode 205: Isomorphic Strings

Problem Restatement We are given two strings: s t We need to determine whether the two strings are isomorphic. Two strings are isomorphic when characters from s can be replaced to form t while preserving structure. Rules: Every character in s must map to exactly one character in t . Different characters in s cannot map to the same character in t . A character may map to itself. The...

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LeetCode 32: Longest Valid Parentheses

Problem Restatement We are given a string s containing only two characters: "(" ")" We need to return the length of the longest valid parentheses substring. A valid parentheses substring must be continuous, balanced, and well-formed. Examples of valid strings: "()" "(())" "()()" "(()())" Examples of invalid strings: "(" ")(" "(()" "())" If there is no valid parentheses substring, return 0 . The constraints allow an empty string, and the...

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LeetCode 22: Generate Parentheses

Problem Restatement We are given an integer n . We need to generate all combinations of well-formed parentheses using exactly n pairs. For example, if: n = 3 then every result must contain: 3 opening parentheses 3 closing parentheses and the parentheses must be valid. The problem asks for all valid strings, not just the number of valid strings. The constraint is 1 <= n <= 8 . Input and...

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LeetCode 482: License Key Formatting

Problem Restatement We are given a string s representing a license key and an integer k . The string contains letters, digits, and dashes. We need to reformat it so that: Rule Meaning Remove old dashes Existing dashes do not matter Convert letters All letters become uppercase Regroup characters Groups are separated by dashes Group size Every group has exactly k characters, except maybe the first First group It may...

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LeetCode 257: Binary Tree Paths

Problem Restatement We are given the root of a binary tree. We need to return all paths from the root to every leaf node. A leaf node is a node with no left child and no right child. Each path must be returned as a string using "->" between node values. For example, this path: 1 \ 2 \ 5 becomes: "1->2->5" The problem asks for all root-to-leaf paths in...

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LeetCode 383: Ransom Note

Problem Restatement We are given two strings: ransomNote magazine We need to decide whether ransomNote can be built using letters from magazine . Each letter from magazine can be used at most once. So we need enough copies of every character required by ransomNote . For example, if ransomNote = "aa" and magazine = "ab" , the answer is False , because magazine has only one a . Input and...

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LeetCode 131: Palindrome Partitioning

Problem Restatement We are given a string s . We need to split it into substrings so that every substring is a palindrome. Return all possible palindrome partitionings of s . A palindrome is a string that reads the same forward and backward, such as: "a" "aa" "aba" "racecar" LeetCode states the task as: partition s so every substring of the partition is a palindrome, then return all possible palindrome...

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LeetCode 20: Valid Parentheses

Problem Restatement We are given a string s containing only these six bracket characters: ( ) { } [ ] We need to determine whether the bracket string is valid. A string is valid when: Every opening bracket is closed by the same type of bracket. Opening brackets are closed in the correct order. Every closing bracket has a matching opening bracket of the same type. For example: "()[]{}" is...

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LeetCode 306: Additive Number

Problem Restatement We are given a string num containing only digits. Return true if the digits can be split into an additive sequence. Otherwise, return false . An additive sequence must satisfy three rules: Rule Meaning At least three numbers We need a, b, c , not only a, b Sum rule Every number after the first two equals the sum of the previous two No leading zeros Numbers like...

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LeetCode 481: Magical String

Problem Restatement We are given an integer n . There is a special infinite string s made only of characters '1' and '2' . The first part of the string is: 1221121221221121122... This string is called magical because if we group equal consecutive characters, the group lengths form the same string again. For example: s = 1 22 11 2 1 22 1 22 11 2 11 22 ... length...

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LeetCode 30: Substring with Concatenation of All Words

Problem Restatement We are given a string s and an array words . Every word in words has the same length. We need to find every starting index in s where a substring is made by concatenating all words in words exactly once, in any order, with no extra characters between them. Return the starting indices in any order. For example, if: words = ["ab", "cd", "ef"] then valid concatenations...

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LeetCode 331: Verify Preorder Serialization of a Binary Tree

Problem Restatement We are given a string preorder . The string represents a preorder traversal serialization of a binary tree. Each token is separated by a comma. A non-null node is written as an integer. A null child is written as # . For example: "9,3,4,#,#,1,#,#,2,#,6,#,#" We need to return true if this string is a valid preorder serialization of some binary tree. We are not allowed to reconstruct the...

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LeetCode 17: Letter Combinations of a Phone Number

Problem Restatement We are given a string digits . Each character is a digit from 2 to 9 . Each digit maps to letters like on a phone keypad: Digit Letters 2 abc 3 def 4 ghi 5 jkl 6 mno 7 pqrs 8 tuv 9 wxyz We need to return all possible letter combinations that the digit string could represent. The answer may be returned in any order. The...

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LeetCode 179: Largest Number

Problem Restatement We are given a list of non-negative integers nums . We need to arrange the numbers so that, when they are joined together, they form the largest possible number. The answer may be very large, so we return it as a string instead of an integer. The official problem states this same requirement: arrange non-negative integers to form the largest number and return it as a string. Input...

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LeetCode 127: Word Ladder

Problem Restatement We are given: Name Meaning beginWord Starting word endWord Target word wordList Dictionary of allowed words We may transform one word into another by changing exactly one character. Every transformed word must exist in wordList . We need to return the number of words in the shortest transformation sequence from beginWord to endWord . If no valid sequence exists, return 0 . Examples Example 1: beginWord = "hit"...

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LeetCode 79: Word Search

Problem Restatement We are given an m x n grid of characters called board and a string called word . We need to return True if word can be formed by walking through the grid. The walk must follow these rules: Rule Meaning Sequential letters The first visited cell must match word[0] , the next must match word[1] , and so on Adjacent cells only We may move up, down,...

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LeetCode 14: Longest Common Prefix

Problem Restatement We are given an array of strings strs . We need to find the longest string that appears at the beginning of every string in the array. If there is no common prefix, return an empty string. For example: ["flower", "flow", "flight"] All strings start with: "fl" So the answer is: "fl" The LeetCode statement asks for the longest common prefix string among an array of strings, returning...

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LeetCode 402: Remove K Digits

Problem Restatement We are given a string num representing a non-negative integer and an integer k . We must remove exactly k digits from num . After removing those digits, the remaining digits must stay in their original relative order. Return the smallest possible integer as a string. The result should not contain leading zeroes. If all digits are removed, or the remaining number becomes empty after removing leading zeroes,...

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LeetCode 227: Basic Calculator II

Problem Restatement We are given a string s representing a mathematical expression. The expression contains: Non-negative integers Operators: + , - , * , / Spaces We must evaluate the expression and return the integer result. Division truncates toward zero. LeetCode examples: Input: "3+2*2" Output: 7 Input: " 3/2 " Output: 1 Input: " 3+5 / 2 " Output: 5 The expression is always valid. Input and Output Item Meaning...

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LeetCode 28: Find the Index of the First Occurrence in a String

Problem Restatement We are given two strings: haystack and needle . We need to return the index where needle first appears inside haystack . If needle does not appear inside haystack , return -1 . For this current version of the problem, both strings have length at least 1 , and both contain only lowercase English letters. Input and Output Item Meaning Input Two strings: haystack and needle Output The...

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LeetCode 13: Roman to Integer

Problem Restatement We are given a Roman numeral string s . We need to convert it into an integer. Roman numerals use seven symbols: Symbol Value I 1 V 5 X 10 L 50 C 100 D 500 M 1000 Usually, Roman numerals are written from largest to smallest. For example: VIII = 5 + 1 + 1 + 1 = 8 But there are six subtractive cases: Roman Value...

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LeetCode 451: Sort Characters By Frequency

Problem Restatement We are given a string s . We need to rearrange its characters so that characters with higher frequency appear before characters with lower frequency. The frequency of a character means how many times it appears in the string. If two characters have the same frequency, their relative order does not matter. Any valid order is accepted. For example: s = "tree" The character frequencies are: Character Frequency...

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LeetCode 301: Remove Invalid Parentheses

Problem Restatement We are given a string s containing parentheses and letters. We need to remove the minimum number of invalid parentheses so that the remaining string becomes valid. Return all unique valid strings that can be produced with the minimum number of removals. The answer may be returned in any order. A valid parentheses string means: Every ( has a matching ) . Every ) matches some earlier (...

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LeetCode 12: Integer to Roman

Problem Restatement We are given an integer num . We need to convert it into a Roman numeral string. Roman numerals use seven base symbols: Symbol Value I 1 V 5 X 10 L 50 C 100 D 500 M 1000 The input satisfies: 1 <= num <= 3999 The LeetCode statement also defines the subtractive forms: Value Roman 4 IV 9 IX 40 XL 90 XC 400 CD 900...

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LeetCode 10: Regular Expression Matching

Problem Restatement We are given two strings: s p s is the input string. p is the pattern. The pattern supports two special characters: Pattern symbol Meaning . Matches any single character * Matches zero or more of the preceding element The match must cover the entire input string, not only part of it. This is the key rule in the problem statement. For example: s = "aa" p =...

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LeetCode 8: String to Integer (atoi)

Problem Restatement We are given a string s . We need to implement myAtoi(s) , which converts the beginning of the string into a signed 32-bit integer. The parsing follows these rules: Ignore leading spaces. Read an optional sign, either + or - . Read digits until a non-digit character appears. If no digits are read, return 0 . Clamp the result into the signed 32-bit integer range. The signed...

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LeetCode 6: Zigzag Conversion

Problem Restatement We are given: a string s an integer numRows We write the characters of the string in a zigzag pattern across multiple rows. Then we read the rows from top to bottom. The result is a new string. The official LeetCode statement uses this example: PAYPALISHIRING with: numRows = 3 becomes: P A H N A P L S I I G Y I R Reading row by...

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LeetCode 5: Longest Palindromic Substring

Problem Restatement We are given a string s . We need to return the longest substring that is a palindrome. A palindrome reads the same forward and backward. Examples: "aba" "racecar" "bb" All of these are palindromes. A substring must be contiguous. For example: "aceca" is a substring of "racecar" . But: "rcar" is not contiguous, so it is not a substring. The problem asks for the substring itself, not...

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 151: Reverse Words in a String

Problem Restatement We are given a string s . The string contains words separated by spaces. A word is a sequence of non-space characters. We need to return a new string where the words appear in reverse order. The output must follow two rules: Words are separated by exactly one space. There are no leading or trailing spaces. For example: s = " hello world " The words are: ["hello",...

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 126: Word Ladder II

Problem Restatement We are given three things: Name Meaning beginWord The word where the transformation starts endWord The word we want to reach wordList The dictionary of valid transformed words We need to return all shortest transformation sequences from beginWord to endWord . A valid transformation sequence looks like this: beginWord -> s1 -> s2 -> ... -> endWord Each step must obey two rules: Adjacent words must differ by...

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LeetCode 76: Minimum Window Substring

Problem Restatement We are given two strings: s: str t: str We need to return the shortest substring of s that contains every character from t . Characters must be counted with duplicates. So if: t = "AABC" then a valid window must contain: A: 2 B: 1 C: 1 If no substring of s can contain all characters from t , return: "" The official problem states that s...

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LeetCode 3: Longest Substring Without Repeating Characters

Problem Restatement We are given a string s . We need to find the length of the longest substring that contains no duplicate characters. A substring means a contiguous part of the string. For example, in: s = "pwwkew" "wke" is a substring because its characters appear next to each other. "pwke" is not a substring because it skips one character. It is a subsequence, not a substring. The problem...

Wiki › Algorithms › 01. Searching and Sorting › 4. Tree and Indexed Search ›

SA-IS Suffix Array

SA-IS Suffix Array Construction SA-IS constructs a suffix array in linear time by classifying characters and recursively sorting a reduced problem of LMS substrings. It is one of the most efficient practical algorithms for suffix array construction. Problem Given a string $T$ of length $n$, construct its suffix array: $$ SA[0 \dots n-1] $$ such that suffixes of $T$ are in lexicographic order. Key Idea Each position in the string...

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Trie Search

Trie Search Trie search locates a key by following a path defined by its characters. Each edge corresponds to one symbol from the alphabet. A node represents a prefix of stored keys. Search cost depends on the key length, not on the number of stored keys. Problem Given a trie root and a string key s , determine whether s exists as a stored key. Return true if present, otherwise...

Wiki › Algorithms › 01. Searching and Sorting › 4. Tree and Indexed Search ›

Suffix Array Doubling Sort

Suffix Array Doubling Sort Suffix array doubling sort constructs a suffix array by iteratively sorting prefixes of length $1, 2, 4, 8, \dots$ until all suffixes are uniquely ranked. It replaces full suffix comparisons with rank comparisons that double in length each round. Problem Given a string $T$ of length $n$, construct a suffix array: $$ SA[0 \dots n-1] $$ such that suffixes of $T$ are sorted lexicographically. Idea Each...

Wiki › Algorithms › 01. Searching and Sorting › 4. Tree and Indexed Search ›

Compressed Trie Search

Compressed Trie Search Compressed trie search locates a string key in a trie where chains of single child nodes are collapsed into one edge. Instead of storing one character per edge, a compressed trie stores a string fragment on each edge. This reduces memory usage and shortens traversal when many keys share long prefixes. Problem Given a compressed trie root root and a string key s , determine whether s...

Wiki › Algorithms › 01. Searching and Sorting › 4. Tree and Indexed Search ›

Radix Tree Search

Radix Tree Search Radix tree search looks up a key in a compact prefix tree where edges store variable length substrings. It is closely related to compressed tries. The main difference is that radix trees are typically implemented for byte sequences or strings and optimize memory layout and lookup performance. Problem Given a radix tree root and a key s , determine whether s exists as a stored key. Return...

Wiki › Algorithms › 01. Searching and Sorting › 4. Tree and Indexed Search ›

Ternary Search Tree Search

Ternary Search Tree Search Ternary search tree search stores strings in a tree where each node contains one character and has three children. The left child stores smaller characters, the middle child continues the current string, and the right child stores larger characters. This combines properties of binary search trees and tries. Problem Given a ternary search tree root root and a string key s , determine whether s is...

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DC3 Suffix Array

Skew Suffix Array Sort (DC3) DC3, also called skew algorithm, builds a suffix array in linear time by splitting suffix positions into residue classes modulo 3 and recursively sorting a subset. It reduces the full suffix ordering problem into a smaller problem, then merges results using rank comparisons. Problem Given a string $T$ of length $n$, construct its suffix array: $$ SA[0 \dots n-1] $$ such that all suffixes are...

Wiki › Algorithms › 01. Searching and Sorting › 4. Tree and Indexed Search ›

Suffix Tree Search

Suffix Tree Search Suffix tree search finds whether a pattern occurs in a text by traversing a compressed trie of all suffixes of the text. A suffix tree stores every suffix of a string. Each edge represents a substring of the original text. This allows pattern matching in time proportional to the pattern length, independent of the text size. Problem Given a suffix tree built from a text $T$ and...

Wiki › Algorithms › 01. Searching and Sorting › 4. Tree and Indexed Search ›

LCP Accelerated Suffix Search

LCP Accelerated Suffix Search LCP accelerated suffix search improves suffix array binary search by using the Longest Common Prefix information to avoid repeated character comparisons. Instead of comparing the pattern against suffixes from scratch at every step, the algorithm reuses previously computed prefix matches. Problem Given a text $T$, suffix array SA , LCP array LCP , and a pattern $P$, determine whether $P$ occurs in $T$. Return true if...

Wiki › Algorithms › 01. Searching and Sorting › 4. Tree and Indexed Search ›

Suffix Array Binary Search

Suffix Array Binary Search Suffix array binary search finds a pattern in a text by searching over the sorted list of suffixes. A suffix array stores the starting positions of all suffixes of a text in lexicographic order. Since the suffixes are sorted, binary search can locate the range of suffixes that begin with a given pattern. Problem Given a text $T$, its suffix array SA , and a pattern...

Wiki › Algorithms › 01. Searching and Sorting › 4. Tree and Indexed Search ›

DC7 Suffix Array

DC7 Suffix Array Sort DC7 is a generalization of DC3 suffix array construction. It splits suffix positions into residue classes modulo 7 and recursively sorts a subset of suffixes, then induces the remaining order. The goal is to reduce comparison depth by encoding suffixes into fixed length tuples and merging sorted partitions. Problem Given a string $T$ of length $n$, construct its suffix array: $$ SA[0 \dots n-1] $$ such...

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LeetCode 3438 - Find Valid Pair of Adjacent Digits in String

LeetCode 3675 - Minimum Operations to Transform String

LeetCode 3615 - Longest Palindromic Path in Graph

LeetCode 3602 - Hexadecimal and Hexatrigesimal Conversion

LeetCode 3628 - Maximum Number of Subsequences After One Inserting

LeetCode 3889 - Mirror Frequency Distance

LeetCode 3884 - First Matching Character From Both Ends

LeetCode 3784 - Minimum Deletion Cost to Make All Characters Equal

LeetCode 3781 - Maximum Score After Binary Swaps

LeetCode 3775 - Reverse Words With Same Vowel Count

LeetCode 3864 - Minimum Cost to Partition a Binary String

LeetCode 3773 - Maximum Number of Equal Length Runs

LeetCode 3863 - Minimum Operations to Sort a String

LeetCode 3860 - Unique Email Groups

LeetCode 3856 - Trim Trailing Vowels

LeetCode 3853 - Merge Close Characters

LeetCode 3758 - Convert Number Words to Digits

LeetCode 3846 - Total Distance to Type a String Using One Finger

LeetCode 3844 - Longest Almost-Palindromic Substring

LeetCode 3756 - Concatenate Non-Zero Digits and Multiply by Sum II

LeetCode 3750 - Minimum Number of Flips to Reverse Binary String

LeetCode 3841 - Palindromic Path Queries in a Tree

LeetCode 3839 - Number of Prefix Connected Groups

LeetCode 3838 - Weighted Word Mapping

LeetCode 3749 - Evaluate Valid Expressions

LeetCode 3746 - Minimum String Length After Balanced Removals

LeetCode 3803 - Count Residue Prefixes

LeetCode 3823 - Reverse Letters Then Special Characters in a String

LeetCode 3817 - Good Indices in a Digit String

LeetCode 3816 - Lexicographically Smallest String After Deleting Duplicate Characters

LeetCode 3813 - Vowel-Consonant Score

LeetCode 3720 - Lexicographically Smallest Permutation Greater Than Target

LeetCode 3714 - Longest Balanced Substring II

LeetCode 3805 - Count Caesar Cipher Pairs

LeetCode 3799 - Word Squares II

LeetCode 3696 - Maximum Distance Between Unequal Words in Array I

LeetCode 3692 - Majority Frequency Characters

LeetCode 3794 - Reverse String Prefix

LeetCode 3703 - Remove K-Balanced Substrings

LeetCode 3557 - Find Maximum Number of Non Intersecting Substrings

LeetCode 3556 - Sum of Largest Prime Substrings

LeetCode 3519 - Count Numbers with Non-Decreasing Digits

LeetCode 3522 - Calculate Score After Performing Instructions

LeetCode 3504 - Longest Palindrome After Substring Concatenation II

LeetCode 3445 - Maximum Difference Between Even and Odd Frequency II

LeetCode 3412 - Find Mirror Score of a String

LeetCode 3406 - Find the Lexicographically Largest String From the Box II

LeetCode 3760 - Maximum Substrings With Distinct Start

LeetCode 3744 - Find Kth Character in Expanded String

LeetCode 3734 - Lexicographically Smallest Palindromic Permutation Greater Than Target

LeetCode 3707 - Equal Score Substrings

LeetCode 3706 - Maximum Distance Between Unequal Words in Array II

LeetCode 3694 - Distinct Points Reachable After Substring Removal

LeetCode 3664 - Two-Letter Card Game

LeetCode 3662 - Filter Characters by Frequency

LeetCode 3612 - Process String with Special Operations I

LeetCode 3614 - Process String with Special Operations II

LeetCode 3606 - Coupon Code Validator

LeetCode 3597 - Partition String

LeetCode 3582 - Generate Tag for Video Caption

LeetCode 3579 - Minimum Steps to Convert String with Operations

LeetCode 3571 - Find the Shortest Superstring II

LeetCode 3563 - Lexicographically Smallest String After Adjacent Removals

LeetCode 3561 - Resulting String After Adjacent Removals

LeetCode 3545 - Minimum Deletions for At Most K Distinct Characters

LeetCode 3541 - Find Most Frequent Vowel and Consonant

LeetCode 3529 - Count Cells in Overlapping Horizontal and Vertical Substrings

LeetCode 3527 - Find the Most Common Response

LeetCode 3518 - Smallest Palindromic Rearrangement II

LeetCode 3517 - Smallest Palindromic Rearrangement I

LeetCode 3503 - Longest Palindrome After Substring Concatenation I

LeetCode 3499 - Maximize Active Section with Trade I

LeetCode 3501 - Maximize Active Section with Trade II

LeetCode 3498 - Reverse Degree of a String

LeetCode 3491 - Phone Number Prefix

LeetCode 3484 - Design Spreadsheet

LeetCode 3481 - Apply Substitutions

LeetCode 3474 - Lexicographically Smallest Generated String

LeetCode 3461 - Check If Digits Are Equal in String After Operations I