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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...

Practice › LeetCode › LeetCode 38xx ›

LeetCode 3890 - Integers With Multiple Sum of Two Cubes

LeetCode Problem 3890 Difficulty: 🟡 Medium Topics: Hash Table, Sorting, Counting, Enumeration Solution Problem Understanding The problem asks us to find all integers x less than or equal to a given integer n such that x can be expressed as the sum of cubes of two positive integers in at least two distinct ways . A “distinct way” means that the pairs (a, b) used to form x are different,...

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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...

Practice › LeetCode › LeetCode 38xx ›

LeetCode 3868 - Minimum Cost to Equalize Arrays Using Swaps

LeetCode Problem 3868 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Counting Solution Problem Understanding The problem requires us to make two integer arrays nums1 and nums2 identical by performing swaps. There are two types of swaps: swaps within the same array, which are free, and swaps between arrays at the same index, which cost 1 per operation. The goal is to determine the minimum cost to make the arrays...

Practice › LeetCode › LeetCode 38xx ›

LeetCode 3866 - First Unique Even Element

LeetCode Problem 3866 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to find the first even integer in an array that appears exactly once . In other words, we need to traverse the array and identify even numbers, count their occurrences, and return the earliest one that occurs only a single time. If there is no such even number, we return -1 ....

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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...

Practice › LeetCode › LeetCode 38xx ›

LeetCode 3852 - Smallest Pair With Different Frequencies

LeetCode Problem 3852 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution LeetCode 3852 - Smallest Pair With Different Frequencies Problem Understanding This problem gives us an integer array nums and asks us to find a pair of distinct values [x, y] that satisfies two conditions: x < y The frequency of x in the array is different from the frequency of y . Among all valid pairs, we must...

Practice › LeetCode › LeetCode 38xx ›

LeetCode 3848 - Check Digitorial Permutation

LeetCode Problem 3848 Difficulty: 🟡 Medium Topics: Math, Counting Solution Problem Understanding The problem asks us to determine whether any permutation of the digits of a given integer n forms a digitorial number. A digitorial number is defined as a number equal to the sum of the factorials of its digits. For instance, 145 is digitorial because $1! + 4! + 5! = 145$. The input is a single integer...

Practice › LeetCode › LeetCode 38xx ›

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...

Practice › LeetCode › LeetCode 38xx ›

LeetCode 3837 - Delayed Count of Equal Elements

LeetCode Problem 3837 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Problem Understanding The problem requires computing a delayed count for each element in an array nums . For an index i , the delayed count is defined as the number of later elements (indices j > i + k ) that are equal to nums[i] . In simpler terms, for each element, we want to count how many...

Practice › LeetCode › LeetCode 37xx ›

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 ,...

Practice › LeetCode › LeetCode 37xx ›

LeetCode 3737 - Count Subarrays With Majority Element I

LeetCode Problem 3737 Difficulty: 🟡 Medium Topics: Array, Hash Table, Divide and Conquer, Segment Tree, Merge Sort, Counting, Prefix Sum Solution Here is a comprehensive, detailed reference guide for LeetCode 3737 - Count Subarrays With Majority Element I , following your formatting requirements exactly. Problem Understanding The problem provides an integer array nums and a target integer. The goal is to count the number of subarrays in which the target...

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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...

Practice › LeetCode › LeetCode 37xx ›

LeetCode 3712 - Sum of Elements With Frequency Divisible by K

LeetCode Problem 3712 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to compute the sum of all elements in the array whose total frequency is divisible by a given integer k . To understand the requirement clearly, we need to focus on two important details: First, we must determine how many times each number appears in the array. This is the element's frequency...

Practice › LeetCode › LeetCode 38xx ›

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...

Practice › LeetCode › LeetCode 36xx ›

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...

Practice › LeetCode › LeetCode 35xx ›

LeetCode 3583 - Count Special Triplets

LeetCode Problem 3583 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Problem Understanding We are given an integer array nums of length n . We must count the number of index triplets (i, j, k) such that 0 <= i < j < k < n and the middle element nums[j] acts as a generator for both ends through the relation nums[i] = 2 * nums[j] and nums[k] =...

Practice › LeetCode › LeetCode 36xx ›

LeetCode 3679 - Minimum Discards to Balance Inventory

LeetCode Problem 3679 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sliding Window, Simulation, Counting Solution LeetCode 3679 - Minimum Discards to Balance Inventory Problem Understanding This problem models a stream of arriving items. On each day, exactly one item arrives, and each item has a type represented by an integer. We are given two parameters: w , the size of the sliding window. m , the maximum number of times...

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LeetCode 3715 - Sum of Perfect Square Ancestors

LeetCode Problem 3715 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Tree, Depth-First Search, Counting, Number Theory Solution Problem Understanding This problem asks us to compute a sum across all non-root nodes in a tree based on ancestor relationships and the property of perfect squares. We are given a rooted tree of n nodes, represented as an edge list. Each node has a positive integer value from the nums array....

Practice › LeetCode › LeetCode 36xx ›

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...

Practice › LeetCode › LeetCode 36xx ›

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...

Practice › LeetCode › LeetCode 36xx ›

LeetCode 3663 - Find The Least Frequent Digit

LeetCode Problem 3663 Difficulty: 🟢 Easy Topics: Array, Hash Table, Math, Counting Solution Problem Understanding The problem asks us to find the least frequent digit in the decimal representation of a given integer n . In other words, we need to count how many times each digit from 0 to 9 appears in the number, and then return the smallest digit that occurs the fewest times. The input is a...

Practice › LeetCode › LeetCode 36xx ›

LeetCode 3636 - Threshold Majority Queries

LeetCode Problem 3636 Difficulty: 🔴 Hard Topics: Array, Hash Table, Binary Search, Divide and Conquer, Counting, Prefix Sum Solution Problem Understanding The problem asks us to process multiple queries on a given array nums where each query specifies a subarray defined by [li, ri] and a threshold . For each query, we need to find an element in the subarray that appears at least threshold times. If multiple elements meet...

Practice › LeetCode › LeetCode 35xx ›

LeetCode 3591 - Check if Any Element Has Prime Frequency

LeetCode Problem 3591 Difficulty: 🟢 Easy Topics: Array, Hash Table, Math, Counting, Number Theory Solution Problem Understanding This problem asks us to determine whether any number in an array appears a prime number of times. In other words, for each unique number in the input array nums , we calculate its frequency-the number of occurrences-and check if that frequency is a prime number. If at least one element has a...

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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 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...

Practice › LeetCode › LeetCode 35xx ›

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...

Practice › LeetCode › LeetCode 34xx ›

LeetCode 3467 - Transform Array by Parity

LeetCode Problem 3467 Difficulty: 🟢 Easy Topics: Array, Sorting, Counting Solution LeetCode 3467 - Transform Array by Parity Problem Understanding The problem gives us an integer array nums and asks us to transform it using three operations that must be performed in a specific order. First, every even number must be replaced with 0 . Second, every odd number must be replaced with 1 . After this transformation, the array...

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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...

Practice › LeetCode › LeetCode 33xx ›

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...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1782 - Count Pairs Of Nodes

LeetCode Problem 1782 Difficulty: 🔴 Hard Topics: Array, Hash Table, Two Pointers, Binary Search, Graph Theory, Sorting, Counting Solution LeetCode 1782 - Count Pairs Of Nodes Problem Understanding We are given an undirected graph with n nodes and a list of edges. Multiple edges between the same pair of nodes are allowed. For any pair of nodes (a, b) , define incident(a, b) as the number of edges connected to...

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Numbers & Mathematics

Numbers & Mathematics Vocabulary Cardinal Numbers Esperanto English Notes nulo 0 zero unu 1 one du 2 two tri 3 three kvar 4 four kvin 5 five ses 6 six sep 7 seven ok 8 eight naŭ 9 nine dek 10 ten dek unu 11 eleven (dek + unu) dek du 12 twelve dek tri 13 thirteen dudek 20 twenty (du + dek) tridek 30 thirty kvardek 40 forty kvindek...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1787 - Make the XOR of All Segments Equal to Zero

LeetCode Problem 1787 Difficulty: 🔴 Hard Topics: Array, Hash Table, Dynamic Programming, Bit Manipulation, Counting Solution Problem Understanding The problem is asking us to modify an array nums such that the XOR of all contiguous subarrays (segments) of length k is equal to zero, while minimizing the number of changes. Each element of nums is a non-negative integer less than $2^{10} = 1024$. The input array has a length up...

Practice › LeetCode › LeetCode 32xx ›

LeetCode 3267 - Count Almost Equal Pairs II

LeetCode Problem 3267 Difficulty: 🔴 Hard Topics: Array, Hash Table, Sorting, Counting, Enumeration Solution LeetCode 3267 - Count Almost Equal Pairs II Problem Understanding We are given an array nums containing positive integers. We need to count how many pairs of indices (i, j) with i < j satisfy a special condition called almost equal . Two numbers x and y are considered almost equal if we can make them...

Practice › LeetCode › LeetCode 22xx ›

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 2225 - Find Players With Zero or One Losses

LeetCode Problem 2225 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting, Counting Solution Problem Understanding The problem gives us a list of match results. Each match is represented as a pair: [winner, loser] This means the player winner defeated the player loser . Our goal is to return two separate lists: All players who never lost any match. All players who lost exactly one match. Both lists must be sorted...

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 3005 - Count Elements With Maximum Frequency

LeetCode Problem 3005 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution LeetCode 3005 - Count Elements With Maximum Frequency Problem Understanding The problem gives us an integer array nums , where every value is positive. We need to determine which elements appear most frequently, then return the total number of occurrences contributed by all such elements. In other words, we first compute the frequency of every distinct number in...

Practice › LeetCode › LeetCode 24xx ›

LeetCode 2453 - Destroy Sequential Targets

LeetCode Problem 2453 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Problem Understanding The problem presents a scenario where we have a list of positive integers nums representing targets placed on a number line. We also have an integer space representing the step interval of a machine. The machine can destroy all targets starting from an initial seed value nums[i] in steps of space . In other words, if...

Practice › LeetCode › LeetCode 19xx ›

LeetCode 1994 - The Number of Good Subsets

LeetCode Problem 1994 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Dynamic Programming, Bit Manipulation, Counting, Number Theory, Bitmask Solution LeetCode 1994 - The Number of Good Subsets Problem Understanding This problem asks us to count how many subsets of the input array have a product that can be written as a multiplication of distinct prime numbers. The phrase "distinct prime numbers" is the key restriction. A product is valid...

Practice › LeetCode › LeetCode 29xx ›

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...

Practice › LeetCode › LeetCode 22xx ›

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 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...

Practice › LeetCode › LeetCode 30xx ›

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...

Practice › LeetCode › LeetCode 24xx ›

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 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 21xx ›

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 25xx ›

LeetCode 2529 - Maximum Count of Positive Integer and Negative Integer

LeetCode Problem 2529 Difficulty: 🟢 Easy Topics: Array, Binary Search, Counting Solution LeetCode 2529 - Maximum Count of Positive Integer and Negative Integer Problem Understanding The problem gives us a sorted integer array nums in non-decreasing order. Our task is to count how many numbers are positive and how many numbers are negative, then return the larger of the two counts. A positive integer is any value greater than 0...

Practice › LeetCode › LeetCode 31xx ›

LeetCode 3185 - Count Pairs That Form a Complete Day II

LeetCode Problem 3185 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to count pairs of elements in an integer array hours such that the sum of the two elements is a multiple of 24 , which we call a "complete day." Each pair (i, j) must satisfy i < j . The input is an array of integers, where each integer represents a...

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LeetCode 2006 - Count Number of Pairs With Absolute Difference K

LeetCode Problem 2006 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem gives us an integer array nums and an integer k . We need to count how many pairs of indices (i, j) satisfy two conditions: i < j |nums[i] - nums[j]| == k The notation |x| represents the absolute value of x , meaning the distance between two numbers regardless of direction. For example,...

Practice › LeetCode › LeetCode 28xx ›

LeetCode 2856 - Minimum Array Length After Pair Removals

LeetCode Problem 2856 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, Binary Search, Greedy, Counting Solution Problem Understanding This problem gives us a sorted integer array nums in non-decreasing order. We may repeatedly remove pairs of elements under one important rule: for a chosen pair (i, j) , the values must satisfy nums[i] < nums[j] . In other words, the two chosen numbers cannot be equal, and the smaller...

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LeetCode 2870 - Minimum Number of Operations to Make Array Empty

LeetCode Problem 2870 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Counting Solution LeetCode 2870 - Minimum Number of Operations to Make Array Empty Problem Understanding The problem gives us an array nums containing positive integers. In a single operation, we are allowed to remove either: Exactly 2 equal elements, or Exactly 3 equal elements. Our goal is to remove all elements from the array using the minimum possible number...

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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...

Practice › LeetCode › LeetCode 24xx ›

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...

Practice › LeetCode › LeetCode 25xx ›

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...

Practice › LeetCode › LeetCode 22xx ›

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 2364 - Count Number of Bad Pairs

LeetCode Problem 2364 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Counting Solution Problem Understanding The problem asks us to count the number of bad pairs in an array. We are given a 0-indexed integer array nums , and a pair of indices (i, j) is considered bad if: i < j j - i != nums[j] - nums[i] In other words, for every pair of indices where the first...

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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 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...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2048 - Next Greater Numerically Balanced Number

LeetCode Problem 2048 Difficulty: 🟡 Medium Topics: Hash Table, Math, Backtracking, Counting, Enumeration Solution Problem Understanding The problem asks us to find the smallest integer strictly greater than n such that the number is numerically balanced. A number is numerically balanced when every digit that appears in the number appears exactly as many times as its value. For example: 22 is balanced because digit 2 appears exactly 2 times. 1333...

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LeetCode 3039 - Apply Operations to Make String Empty

LeetCode Problem 3039 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting, Counting Solution LeetCode 3039 - Apply Operations to Make String Empty Problem Understanding The problem gives us a lowercase string s and defines a repeated operation. During one operation, we scan through all letters from 'a' to 'z' . For each letter, if that letter appears in the current string, we remove its first occurrence. This process continues until...

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LeetCode 2341 - Maximum Number of Pairs in Array

LeetCode Problem 2341 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to take an array of integers, nums , and repeatedly form pairs of equal numbers until no more pairs can be formed. Each operation removes exactly two identical numbers from the array, and we continue doing this until it is no longer possible. The goal is to return a list of two...

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LeetCode 3046 - Split the Array

LeetCode Problem 3046 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to determine if an even-length array can be split into two equal-sized subarrays, each containing only distinct elements . In other words, we must partition the array nums of length n into nums1 and nums2 of size n/2 such that there are no repeated numbers within either subarray. The function should return...

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LeetCode 2183 - Count Array Pairs Divisible by K

LeetCode Problem 2183 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Counting, Number Theory Solution Problem Understanding The problem gives us an integer array nums and an integer k . We must count how many index pairs (i, j) satisfy two conditions: i < j nums[i] * nums[j] is divisible by k In other words, for every pair of distinct elements in the array, we check whether their product contains...

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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...

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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 2404 - Most Frequent Even Element

LeetCode Problem 2404 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to identify the most frequent even number in an integer array nums . If multiple even numbers share the highest frequency, we should return the smallest among them. If the array contains no even numbers, the function should return -1 . The input array nums can have up to 2000 elements, each...

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LeetCode 1982 - Find Array Given Subset Sums

LeetCode Problem 1982 Difficulty: 🔴 Hard Topics: Array, Hash Table, Sorting, Counting Solution Problem Understanding The problem gives us an integer n and an array sums of length 2^n , which contains every possible subset sum of an unknown array of length n . Our task is to reconstruct any one valid array ans of size n such that the multiset of all subset sums of ans exactly matches the...

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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 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 3128 - Right Triangles

LeetCode Problem 3128 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Combinatorics, Counting Solution Problem Understanding The problem gives us a binary matrix grid , where each cell contains either 0 or 1 . We need to count how many valid right triangles can be formed using cells whose value is 1 . A valid right triangle in this problem is defined by selecting exactly three cells containing 1 such...

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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 2148 - Count Elements With Strictly Smaller and Greater Elements

LeetCode Problem 2148 Difficulty: 🟢 Easy Topics: Array, Sorting, Counting Solution Problem Understanding The problem gives us an integer array nums and asks us to count how many elements satisfy a very specific condition. For an element nums[i] to be counted, there must exist: at least one element in the array that is strictly smaller than nums[i] at least one element in the array that is strictly greater than nums[i]...

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LeetCode 2206 - Divide Array Into Equal Pairs

LeetCode Problem 2206 Difficulty: 🟢 Easy Topics: Array, Hash Table, Bit Manipulation, Counting Solution Problem Understanding The problem gives us an integer array nums containing exactly 2 * n elements. Our task is to determine whether it is possible to split the array into n valid pairs, where each pair contains two identical numbers. In simpler terms, every number in the array must be matched with another occurrence of the...

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LeetCode 3265 - Count Almost Equal Pairs I

LeetCode Problem 3265 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting, Counting, Enumeration Solution Problem Understanding The problem asks us to find the number of index pairs (i, j) in an array nums such that i < j and the integers nums[i] and nums[j] are almost equal . Two integers are considered almost equal if you can make them equal by swapping any two digits at most once in either...

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LeetCode 2960 - Count Tested Devices After Test Operations

LeetCode Problem 2960 Difficulty: 🟢 Easy Topics: Array, Simulation, Counting Solution Problem Understanding This problem asks us to simulate a sequence of device tests based on battery percentages. We are given a 0-indexed integer array batteryPercentages , where each element represents the current battery level of a device. We process the devices from left to right, starting at index 0 and ending at index n - 1 . For each...

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LeetCode 2547 - Minimum Cost to Split an Array

LeetCode Problem 2547 Difficulty: 🔴 Hard Topics: Array, Hash Table, Dynamic Programming, Counting Solution Problem Understanding The problem asks us to split the array nums into one or more contiguous non-empty subarrays such that the total cost is minimized. For every subarray, we define a special quantity called its importance value. To compute it, we first construct the trimmed version of the subarray by removing every element that appears exactly...

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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 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 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...

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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 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 2275 - Largest Combination With Bitwise AND Greater Than Zero

LeetCode Problem 2275 Difficulty: 🟡 Medium Topics: Array, Hash Table, Bit Manipulation, Counting Solution Problem Understanding The problem asks us to find the largest possible group of numbers from the candidates array such that the bitwise AND of every number in that group is greater than 0 . A bitwise AND operation only keeps bits that are set to 1 in every participating number. For example: 6 = 110 14...

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LeetCode 2499 - Minimum Total Cost to Make Arrays Unequal

LeetCode Problem 2499 Difficulty: 🔴 Hard Topics: Array, Hash Table, Greedy, Counting Solution Problem Understanding You are given two arrays, nums1 and nums2 , both of length n . You are allowed to perform operations only on nums1 . In a single operation, you may swap any two indices in nums1 , and the cost of that operation is the sum of the two indices involved in the swap. The...

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LeetCode 3312 - Sorted GCD Pair Queries

LeetCode Problem 3312 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Binary Search, Combinatorics, Counting, Number Theory, Prefix Sum Solution === 1996-G1 === IMO 1996 Shortlist G1 Origin: GBR Problem Let triangle $ABC$ have orthocenter $H$, and let $P$ be a point on its circumcircle, distinct from $A$, $B$, $C$. Let $E$ be the foot of the altitude $BH$, let $PAQB$ and $PARC$ be parallelograms, and let $AQ$ meet $HR$...

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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...

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LeetCode 2248 - Intersection of Multiple Arrays

LeetCode Problem 2248 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sorting, Counting Solution Problem Understanding The problem is asking us to find the intersection of multiple arrays. Specifically, given a 2D array nums , where each nums[i] is a non-empty array of distinct positive integers , we want to identify which integers appear in every array in nums . The result should be sorted in ascending order. In simpler terms,...

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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...

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LeetCode 2244 - Minimum Rounds to Complete All Tasks

LeetCode Problem 2244 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Counting Solution Here is a complete, detailed technical solution guide for LeetCode 2244 - Minimum Rounds to Complete All Tasks following your formatting and content requirements. Problem Understanding The problem provides a 0-indexed array tasks , where each element represents a task's difficulty level. Each round, you are allowed to complete exactly 2 or 3 tasks of the same...

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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 2190 - Most Frequent Number Following Key In an Array

LeetCode Problem 2190 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to find the integer that appears most frequently immediately after a given key value in the array. More specifically, we are given an integer array nums and an integer key , which is guaranteed to exist in the array. We need to examine every occurrence of key in nums . Whenever we...

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LeetCode 2347 - Best Poker Hand

LeetCode Problem 2347 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem gives us exactly five playing cards. Each card is represented by two pieces of information: ranks[i] represents the numerical rank of the card, from 1 to 13 suits[i] represents the suit of the card, using characters from 'a' to 'd' We need to determine the strongest poker hand that can be formed from these...

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LeetCode 3153 - Sum of Digit Differences of All Pairs

LeetCode Problem 3153 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Counting Solution Problem Understanding The problem asks us to compute the total digit difference across every pair of numbers in the array. All numbers have the same number of digits. For any two numbers, their digit difference is defined as the number of positions where the digits are different. For example: 13 and 23 differ only at the first...

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LeetCode 1854 - Maximum Population Year

LeetCode Problem 1854 Difficulty: 🟢 Easy Topics: Array, Counting, Prefix Sum Solution Problem Understanding The problem asks us to determine the year with the highest population given a list of birth and death years. Each element in the input logs is a pair [birthi, deathi] representing the inclusive start of life at birthi and exclusive end at deathi . In other words, a person is considered alive from their birth...

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LeetCode 2442 - Count Number of Distinct Integers After Reverse Operations

LeetCode Problem 2442 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Counting Solution Problem Understanding The problem asks us to compute the number of distinct integers in an array after performing a specific operation on each element: reversing its digits and adding it to the array. In other words, for every integer in nums , we generate its reverse and append it. After processing all original elements, we count how...

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LeetCode 2526 - Find Consecutive Integers from a Data Stream

LeetCode Problem 2526 Difficulty: 🟡 Medium Topics: Hash Table, Design, Queue, Counting, Data Stream Solution Problem Understanding The problem asks us to design a data structure that continuously processes a stream of integers and determines whether the most recent k integers are all equal to a specific target value. We are given two parameters when initializing the data structure: value , which represents the integer we are interested in tracking....

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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"...

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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...

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LeetCode 3044 - Most Frequent Prime

LeetCode Problem 3044 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Matrix, Counting, Enumeration, Number Theory Solution Problem Understanding The problem asks us to traverse a m x n integer matrix and generate numbers by moving in straight lines along eight possible directions: east, south-east, south, south-west, west, north-west, north, and north-east. Starting from any cell, we can append the digits along a chosen path to form numbers. Importantly, numbers...

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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"...

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LeetCode 3071 - Minimum Operations to Write the Letter Y on a Grid

LeetCode Problem 3071 Difficulty: 🟡 Medium Topics: Array, Hash Table, Matrix, Counting Solution Problem Understanding The problem asks us to determine the minimum number of operations to convert a given n x n grid into a representation of the letter Y, under specific conditions. The grid contains integers 0, 1, or 2. The cells that belong to the Y consist of two diagonals starting from the top corners to the...

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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...

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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 3063 - Linked List Frequency

LeetCode Problem 3063 Difficulty: 🟢 Easy Topics: Hash Table, Linked List, Counting Solution Problem Understanding This problem asks us to analyze a singly-linked list of integers and produce a new linked list containing the frequencies of each distinct element in the input list. Specifically, given the head of a linked list containing k distinct elements, we are to return another linked list of length k where each node represents the...

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LeetCode 2198 - Number of Single Divisor Triplets

LeetCode Problem 2198 Difficulty: 🟡 Medium Topics: Array, Counting, Enumeration Solution Problem Understanding The problem asks us to count how many ordered triplets of distinct indices (i, j, k) satisfy a very specific divisibility condition. For every triplet, we compute: $$\text{sum} = nums[i] + nums[j] + nums[k]$$ The triplet is considered valid if this sum is divisible by exactly one of the three participating values: nums[i] nums[j] nums[k] The indices...

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LeetCode 3186 - Maximum Total Damage With Spell Casting

LeetCode Problem 3186 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, Binary Search, Dynamic Programming, Sorting, Counting Solution Problem Understanding The problem gives us an array power , where each element represents the damage value of a spell. Every spell can be used at most once, and multiple spells may share the same damage value. The restriction is the important part of the problem. If we decide to cast...

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LeetCode 3184 - Count Pairs That Form a Complete Day I

LeetCode Problem 3184 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to find all pairs of indices (i, j) in an array hours such that i < j and the sum of hours[i] + hours[j] is an exact multiple of 24, which is considered a "complete day." In simpler terms, we are trying to count all unique pairs of hours that add up...

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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 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 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 2150 - Find All Lonely Numbers in the Array

LeetCode Problem 2150 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution LeetCode 2147 - Number of Ways to Divide a Long Corridor Problem Understanding The corridor is represented as a string where each character is either 'S' for a seat or 'P' for a plant. We already have fixed dividers at both ends of the corridor, and we may optionally place additional dividers between adjacent positions. The goal is...

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LeetCode 1862 - Sum of Floored Pairs

LeetCode Problem 1862 Difficulty: 🔴 Hard Topics: Array, Math, Binary Search, Counting, Enumeration, Prefix Sum Solution Problem Understanding The problem asks us to calculate the sum of the integer division results, floor(nums[i] / nums[j]) , for every pair of elements (i, j) in a given array nums . Here, floor() represents the largest integer less than or equal to the division result. The array nums can have up to 10^5...

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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 1940 - Longest Common Subsequence Between Sorted Arrays

LeetCode Problem 1940 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to find the longest common subsequence (LCS) among multiple sorted integer arrays. In simpler terms, we are given a list of arrays where each array is sorted in strictly increasing order, and we need to identify the sequence of numbers that appears in all arrays in the same relative order and is...

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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 2782 - Number of Unique Categories

LeetCode Problem 2782 Difficulty: 🟡 Medium Topics: Union-Find, Interactive, Counting Solution Problem Understanding This problem asks us to determine how many distinct categories exist among n elements. The elements are labeled from 0 to n - 1 , but we are not given the category values directly. Instead, we interact with the hidden categorization through the provided API: haveSameCategory(a, b) This function tells us whether two elements belong to the...

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LeetCode 2748 - Number of Beautiful Pairs

LeetCode Problem 2748 Difficulty: 🟢 Easy Topics: Array, Hash Table, Math, Counting, Number Theory Solution LeetCode 2748 - Number of Beautiful Pairs Problem Understanding The problem gives us a 0-indexed integer array nums . We must count how many pairs of indices (i, j) satisfy 0 <= i < j < n and have the following property: Take the first digit of nums[i] . Take the last digit of nums[j]...

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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 2170 - Minimum Operations to Make the Array Alternating

LeetCode Problem 2170 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Counting Solution Problem Understanding The problem requires transforming a given integer array nums into an alternating array with the minimum number of operations. An array is alternating if elements at even indices repeat every two steps ( nums[i] == nums[i-2] for i >= 2 ) and elements at odd indices differ from their previous element ( nums[i] != nums[i-1]...

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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 3371 - Identify the Largest Outlier in an Array

LeetCode Problem 3371 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting, Enumeration Solution Problem Understanding The problem gives us an integer array nums containing exactly three types of elements: n - 2 special numbers One element equal to the sum of all special numbers One outlier The task is to return the largest possible value that could serve as the outlier. The important detail is that these roles are determined...

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LeetCode 3238 - Find the Number of Winning Players

LeetCode Problem 3238 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution LeetCode 3238 - Find the Number of Winning Players Problem Understanding The problem gives us n players and a list called pick , where each entry is of the form [xi, yi] . This means player xi picked a ball with color yi . A player wins if they have picked strictly more than their player index number...

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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,...

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...

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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 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 18xx ›

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...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 914 - X of a Kind in a Deck of Cards

LeetCode Problem 914 Difficulty: 🟢 Easy Topics: Array, Hash Table, Math, Counting, Number Theory Solution Problem Understanding The problem asks whether it is possible to partition a deck of cards, represented as an integer array deck , into groups such that each group contains exactly x cards, all of which have the same integer value, and x is greater than 1. Essentially, we need to find a number x that...

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 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...

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...

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LeetCode 2013 - Detect Squares

LeetCode Problem 2013 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Counting, Data Stream Solution Problem Understanding The problem presents a stream of points in a 2D plane and asks us to design a data structure that supports two operations efficiently: adding points and counting squares. The add operation inserts a point into the data structure, and the count operation calculates the number of axis-aligned squares that can be formed...

Practice › LeetCode › LeetCode 14xx ›

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,...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1814 - Count Nice Pairs in an Array

LeetCode Problem 1814 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Counting Solution Problem Understanding The problem gives us an array of non-negative integers called nums . We need to count how many index pairs (i, j) satisfy the following conditions: i < j nums[i] + rev(nums[j]) == nums[j] + rev(nums[i]) Here, rev(x) means reversing the digits of the integer. For example: rev(123) = 321 rev(120) = 21 The goal...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1519 - Number of Nodes in the Sub-Tree With the Same Label

LeetCode Problem 1519 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search, Counting Solution Problem Understanding The problem gives us a tree with n nodes numbered from 0 to n - 1 . The tree is represented by an array of edges , where each edge connects two nodes ai and bi . Each node also has a label, represented as a string labels , where labels[i] is...

Practice › LeetCode › LeetCode 14xx ›

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...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 229 - Majority Element II

LeetCode Problem 229 Difficulty: 🟡 Medium Topics: Array, Hash Table, Sorting, Counting Solution Problem Understanding The problem asks us to find every element in an integer array that appears more than ⌊ n/3 ⌋ times, where n is the length of the array. The floor notation means we round down to the nearest integer. For example, if the array length is 8, then ⌊ 8/3 ⌋ = 2 , so...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1054 - Distant Barcodes

LeetCode Problem 1054 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sorting, Heap (Priority Queue), Counting Solution Problem Understanding The problem is asking us to rearrange a list of barcodes so that no two adjacent barcodes are the same. The input is an array barcodes of integers where each integer represents a type of barcode. The output should be a rearranged array that satisfies the adjacency constraint. The constraints specify...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1213 - Intersection of Three Sorted Arrays

LeetCode Problem 1213 Difficulty: 🟢 Easy Topics: Array, Hash Table, Binary Search, Counting Solution Problem Understanding The problem gives us three integer arrays, arr1 , arr2 , and arr3 . Each array is already sorted in strictly increasing order. Strictly increasing means there are no duplicate values inside the same array, and every next element is larger than the previous one. Our task is to return a new sorted array...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1748 - Sum of Unique Elements

LeetCode Problem 1748 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem gives us an integer array nums and asks us to compute the sum of all elements that appear exactly once in the array. An element is considered unique only if its frequency is exactly one. If a value appears two or more times, it must not contribute to the final sum at all. For...

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 10xx ›

LeetCode 1010 - Pairs of Songs With Total Durations Divisible by 60

LeetCode Problem 1010 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to find pairs of songs whose total duration is divisible by 60. Specifically, we are given an array time where each element represents the length of a song in seconds. We need to count all unique pairs (i, j) where i < j and (time[i] + time[j]) % 60 == 0 ....

Practice › LeetCode › LeetCode 20xx ›

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 2025 - Maximum Number of Ways to Partition an Array

LeetCode Problem 2025 Difficulty: 🔴 Hard Topics: Array, Hash Table, Counting, Enumeration, Prefix Sum Solution Problem Understanding The problem asks us to count how many valid partition points exist in an array after optionally changing at most one element to a given value k . A partition is defined by a pivot index pivot such that: 1 <= pivot < n The sum of elements on the left side equals...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1128 - Number of Equivalent Domino Pairs

LeetCode Problem 1128 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding The problem is asking us to count all pairs of dominoes in a list that are equivalent, where two dominoes [a, b] and [c, d] are considered equivalent if one is a rotation of the other, meaning either (a == c and b == d) or (a == d and b == c) . In other...

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 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 09xx ›

LeetCode 900 - RLE Iterator

LeetCode Problem 900 Difficulty: 🟡 Medium Topics: Array, Design, Counting, Iterator Solution Problem Understanding This problem asks us to design an iterator over a run-length encoded sequence instead of storing the full sequence explicitly. In run-length encoding, the array is organized in pairs: encoding[i] represents how many times a value appears encoding[i + 1] represents the value itself For example: [3,8,2,5] means: 8 appears 3 times 5 appears 2 times...

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 09xx ›

LeetCode 992 - Subarrays with K Different Integers

LeetCode Problem 992 Difficulty: 🔴 Hard Topics: Array, Hash Table, Sliding Window, Counting Solution Problem Understanding The problem asks us to count how many contiguous subarrays contain exactly k distinct integers. We are given: An integer array nums An integer k A subarray is any continuous portion of the array. For each possible subarray, we need to determine whether it contains exactly k unique values. If it does, it contributes...

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 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 01xx ›

LeetCode 169 - Majority Element

LeetCode Problem 169 Difficulty: 🟢 Easy Topics: Array, Hash Table, Divide and Conquer, Sorting, Counting Solution Problem Understanding The problem asks us to identify the majority element in a given array of integers nums . The majority element is defined as the number that appears more than half of the times in the array, i.e., more than ⌊n / 2⌋ times where n is the length of the array. The...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1512 - Number of Good Pairs

LeetCode Problem 1512 Difficulty: 🟢 Easy Topics: Array, Hash Table, Math, Counting Solution LeetCode 1512 - Number of Good Pairs Problem Understanding The problem gives us an integer array nums and asks us to count how many pairs of indices (i, j) satisfy two conditions: nums[i] == nums[j] i < j Such pairs are called "good pairs". In simpler terms, we need to count how many times the same value...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1198 - Find Smallest Common Element in All Rows

LeetCode Problem 1198 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Matrix, Counting Solution Problem Understanding The problem asks us to find the smallest element that appears in every row of a given m x n matrix mat , where each row is sorted in strictly increasing order. In other words, we need an element that is common to all rows and is the minimum among all such elements....

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 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 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 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" ,...

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LeetCode 1655 - Distribute Repeating Integers

LeetCode Problem 1655 Difficulty: 🔴 Hard Topics: Array, Hash Table, Dynamic Programming, Backtracking, Bit Manipulation, Counting, Bitmask Solution Problem Understanding This problem asks whether we can satisfy a set of customer requests using repeated integers from the array nums . Each customer wants a certain quantity of numbers, given by quantity[i] . The important restriction is that every number given to a single customer must be identical. A customer asking...

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 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 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 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 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"...

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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 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 03xx ›

LeetCode 347 - Top K Frequent Elements

LeetCode Problem 347 Difficulty: 🟡 Medium Topics: Array, Hash Table, Divide and Conquer, Sorting, Heap (Priority Queue), Bucket Sort, Counting, Quickselect Solution Problem Understanding The problem asks us to return the k most frequently occurring elements from an integer array. We are given an array nums , which may contain duplicates, and an integer k . Our task is to identify which values appear most often and return exactly k...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 945 - Minimum Increment to Make Array Unique

LeetCode Problem 945 Difficulty: 🟡 Medium Topics: Array, Greedy, Sorting, Counting Solution Problem Understanding This problem asks us to take an integer array nums and perform a series of increment operations so that every element in the array becomes unique. An increment operation increases an element by exactly 1. The goal is to compute the minimum number of such moves required. In simpler terms, we are asked: given an array...

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,...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1603 - Design Parking System

LeetCode Problem 1603 Difficulty: 🟢 Easy Topics: Design, Simulation, Counting Solution Problem Understanding This problem asks us to design a very small parking lot management system. The parking lot contains exactly three types of parking spaces: Big parking spaces Medium parking spaces Small parking spaces The constructor receives the number of available spaces for each type. After initialization, cars arrive one at a time, and for every arriving car we...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 621 - Task Scheduler

LeetCode Problem 621 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sorting, Heap (Priority Queue), Counting Solution Problem Understanding The Task Scheduler problem asks us to determine the minimum number of CPU intervals required to complete a list of tasks with a cooling constraint. Each task is represented by an uppercase letter A-Z. The CPU can execute one task per interval or remain idle. After executing a task, the CPU...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1394 - Find Lucky Integer in an Array

LeetCode Problem 1394 Difficulty: 🟢 Easy Topics: Array, Hash Table, Counting Solution Problem Understanding This problem asks us to identify a special number in an array called a lucky integer . A lucky integer is defined as an integer in the array whose frequency exactly matches its value. For example, if the number 3 appears exactly 3 times in the array, it is considered lucky. Our task is to find...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 594 - Longest Harmonious Subsequence

LeetCode Problem 594 Difficulty: 🟢 Easy Topics: Array, Hash Table, Sliding Window, Sorting, Counting Solution Problem Understanding The problem asks us to find the length of the longest harmonious subsequence in an integer array. A harmonious array is defined as one where the difference between the maximum and minimum values is exactly 1 . In simpler terms, given an array nums , we are searching for the longest collection of...

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...

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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 09xx ›

LeetCode 923 - 3Sum With Multiplicity

LeetCode Problem 923 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, Sorting, Counting Solution Problem Understanding The problem asks us to count how many index triplets (i, j, k) satisfy two conditions: The indices must follow the order i < j < k The values at those indices must sum to the given target Formally, we want: arr[i] + arr[j] + arr[k] == target The important detail is that...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1356 - Sort Integers by The Number of 1 Bits

LeetCode Problem 1356 Difficulty: 🟢 Easy Topics: Array, Bit Manipulation, Sorting, Counting Solution Problem Understanding The problem asks us to sort an integer array arr based on the number of 1 bits in the binary representation of each element. This means that we are not sorting by the integer value itself but by the count of 1 s in its binary form. If two numbers have the same number of...

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LeetCode 1399 - Count Largest Group

LeetCode Problem 1399 Difficulty: 🟢 Easy Topics: Hash Table, Math, Counting Solution Problem Understanding The problem asks us to group all integers from 1 to n based on the sum of their digits. Every number belongs to exactly one group, determined by adding together all of its decimal digits. For example: 14 has digit sum 1 + 4 = 5 5 also has digit sum 5 So both numbers belong...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1497 - Check If Array Pairs Are Divisible by k

LeetCode Problem 1497 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution I can do that, but the full guide at the level of detail you requested, including complete prose explanations, worked traces, Python and Go implementations, comprehensive test cases, and edge-case analysis, will be quite long and may exceed a single message limit. To preserve quality and avoid truncation, I will split it into 2 parts : Part 1...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1726 - Tuple with Same Product

LeetCode Problem 1726 Difficulty: 🟡 Medium Topics: Array, Hash Table, Counting Solution Problem Understanding The problem asks us to count all tuples (a, b, c, d) from a given array of distinct positive integers such that the product of the first two elements equals the product of the second two elements, a * b = c * d , and all elements are distinct. Importantly, permutations of the same numbers...

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...

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LeetCode 1742 - Maximum Number of Balls in a Box

LeetCode Problem 1742 Difficulty: 🟢 Easy Topics: Hash Table, Math, Counting Solution Problem Understanding The problem describes a scenario in which you have a sequence of balls numbered consecutively from lowLimit to highLimit , inclusive. Each ball must be placed into a box, where the box number is determined by the sum of the digits of the ball’s number. For example, ball 321 goes into box 3 + 2 +...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2001 - Number of Pairs of Interchangeable Rectangles

LeetCode Problem 2001 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Counting, Number Theory Solution LeetCode 2001 - Number of Pairs of Interchangeable Rectangles Problem Understanding This problem asks us to count how many pairs of rectangles are interchangeable. Each rectangle is represented by two integers, width and height . Two rectangles are considered interchangeable if their width-to-height ratios are exactly the same. In mathematical terms, rectangles i and j...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2029 - Stone Game IX

LeetCode Problem 2029 Difficulty: 🟡 Medium Topics: Array, Math, Greedy, Counting, Game Theory Solution Problem Understanding The problem describes a turn-based game between Alice and Bob with a row of stones, each having a numeric value. On each turn, a player removes any stone of their choice. The game has a losing condition based on the sum of removed stones: if the cumulative sum of all removed stones becomes divisible...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1481 - Least Number of Unique Integers after K Removals

LeetCode Problem 1481 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sorting, Counting Solution Problem Understanding This problem asks us to minimize the number of unique integers in an array after removing exactly k elements. In other words, we are given a list arr and a number k , and we need to strategically remove k elements so that the count of distinct integers left in the array is as...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1819 - Number of Different Subsequences GCDs

LeetCode Problem 1819 Difficulty: 🔴 Hard Topics: Array, Math, Counting, Number Theory Solution Problem Understanding The problem asks us to compute how many distinct values can appear as the GCD of some non-empty subsequence of the given array. A subsequence is formed by deleting zero or more elements while preserving order. Since the order does not matter for the GCD operation, the real challenge is determining which integers can be...

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 12xx ›

LeetCode 1267 - Count Servers that Communicate

LeetCode Problem 1267 Difficulty: 🟡 Medium Topics: Array, Depth-First Search, Breadth-First Search, Union-Find, Matrix, Counting Solution Problem Understanding The problem gives us a two dimensional grid representing a server center. Each cell contains either: 1 , meaning there is a server in that position 0 , meaning the cell is empty A server can communicate with another server if both servers are located in the same row or the same...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 869 - Reordered Power of 2

LeetCode Problem 869 Difficulty: 🟡 Medium Topics: Hash Table, Math, Sorting, Counting, Enumeration Solution Problem Understanding The problem gives us a positive integer n and asks whether its digits can be rearranged to form a power of two. The rearrangement may keep the digits in their original order or place them in any other order, but the resulting number cannot contain a leading zero. A power of two is any...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1121 - Divide Array Into Increasing Sequences

LeetCode Problem 1121 Difficulty: 🔴 Hard Topics: Array, Counting Solution Problem Understanding The problem asks us to determine if a sorted integer array nums can be divided into one or more disjoint increasing subsequences, where each subsequence has a length of at least k . The array is sorted in non-decreasing order, which means duplicates are allowed and elements do not decrease as we move from left to right. The...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1775 - Equal Sum Arrays With Minimum Number of Operations

LeetCode Problem 1775 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Counting Solution Problem Understanding The problem gives us two integer arrays, nums1 and nums2 , where every element is between 1 and 6 . In one operation, we may choose any element from either array and change it to any value from 1 to 6 . Our goal is to make the total sum of both arrays equal while...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1612 - Check If Two Expression Trees are Equivalent

LeetCode Problem 1612 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Binary Tree, Counting Solution Problem Understanding The problem asks us to determine whether two binary expression trees are equivalent, where equivalence is defined as the trees representing the same arithmetic expression, modulo the order of addition since addition is commutative. Each tree node contains either a '+' operator (for internal nodes) or a variable (for leaf nodes). The...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1090 - Largest Values From Labels

LeetCode Problem 1090 Difficulty: 🟡 Medium Topics: Array, Hash Table, Greedy, Sorting, Counting Solution Problem Understanding The problem gives us two arrays, values and labels , where each index represents a single item. The item at index i has a value values[i] and a category or group identifier labels[i] . We want to choose a subset of these items such that two constraints are satisfied: We can select at most...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1090: Largest Values From Labels

Problem Restatement We have n items, each with a value and a label . We want to choose a subset of items to maximize total value, subject to: At most numWanted items can be chosen. At most useLimit items with the same label can be chosen. The official constraints state that n == values.length == labels.length and 1 <= numWanted, useLimit <= n <= 2 * 10^4 . Input and...

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 1010: Pairs of Songs With Total Durations Divisible by 60

Problem Restatement We are given a list of song durations time . We want to count pairs of songs (i, j) where i < j and: (time[i] + time[j]) % 60 == 0 We need to return the number of such pairs. The official constraints state that 1 <= time.length <= 6 * 10^4 and 1 <= time[i] <= 500 . Input and Output Item Meaning Input Array time of...

Practice › LeetCode › LeetCode (archived) › LeetCode 09xx ›

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 08xx ›

LeetCode 825: Friends Of Appropriate Ages

Problem Restatement We are given an array ages , where ages[i] is the age of the i th person. A person x will not send a friend request to person y if x == y , or if any of these conditions is true: age[y] <= 0.5 * age[x] + 7 age[y] > age[x] age[y] > 100 and age[x] < 100 Otherwise, x sends a friend request to y ....

Practice › LeetCode › LeetCode (archived) › LeetCode 09xx ›

LeetCode 997: Find the Town Judge

Problem Restatement There are n people in a town labeled from 1 to n . We are given an array trust , where each pair: [a, b] means person a trusts person b . We need to find the town judge. If the judge exists, then: Rule Meaning Trusts nobody The judge has no outgoing trust Trusted by everyone else Exactly n - 1 people trust the judge Unique There...

Practice › LeetCode › LeetCode (archived) › LeetCode 09xx ›

LeetCode 996: Number of Squareful Arrays

Problem Restatement We are given an integer array nums . An array is squareful if the sum of every pair of adjacent elements is a perfect square. We need to return the number of unique permutations of nums that are squareful. For example, if two adjacent values are 1 and 8 , then: 1 + 8 = 9 Since 9 is a perfect square, this adjacent pair is valid. The...

Practice › LeetCode › LeetCode (archived) › LeetCode 09xx ›

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 08xx ›

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 09xx ›

LeetCode 992: Subarrays with K Different Integers

Problem Restatement We are given an integer array nums and an integer k . A good subarray is a contiguous subarray that contains exactly k different integers. We need to return the number of good subarrays. For example: nums = [1, 2, 1, 2, 3] k = 2 The good subarrays are: [1, 2] [2, 1] [1, 2] [2, 3] [1, 2, 1] [2, 1, 2] [1, 2, 1, 2]...

Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 750: Number Of Corner Rectangles

Problem Restatement We are given an m x n binary matrix grid . Each cell is either 0 or 1 . We need to count how many axis-aligned rectangles can be formed where all four corner cells are 1 . Only the corners need to be 1 . The cells inside the rectangle and along its edges may be either 0 or 1 . All four corner cells must be...

Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 06xx ›

LeetCode 621: Task Scheduler

Problem Restatement We are given a list of CPU tasks. Each task is represented by a capital letter from A to Z . Each task takes exactly one CPU interval. The CPU can do one of two things in each interval: Execute one task. Stay idle. There is also a non-negative integer n . This means that between two executions of the same task, there must be at least n...

Practice › LeetCode › LeetCode (archived) › LeetCode 06xx ›

LeetCode 697: Degree of an Array

Problem Restatement We are given a non-empty array nums . The degree of an array is the maximum frequency of any element in that array. We need to find the smallest length of a contiguous subarray that has the same degree as the whole array. If several elements have the maximum frequency, we can choose the one that gives the shortest span. The official statement defines the degree as the...

Practice › LeetCode › LeetCode (archived) › LeetCode 06xx ›

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 08xx ›

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 08xx ›

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 09xx ›

LeetCode 982: Triples with Bitwise AND Equal To Zero

Problem Restatement We are given an integer array nums . We need to count the number of ordered triples of indices (i, j, k) such that: nums[i] & nums[j] & nums[k] == 0 The indices may repeat. For example, (0, 0, 1) is allowed. The triple is ordered, so (0, 1, 2) and (1, 0, 2) are counted as different triples if both satisfy the condition. The official constraints are:...

Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

LeetCode 594: Longest Harmonious Subsequence

Problem Restatement We are given an integer array nums . A harmonious subsequence is a subsequence where the difference between the maximum value and the minimum value is exactly 1 . We need to return the length of the longest harmonious subsequence. A subsequence is formed by deleting some elements, possibly none, without changing the order of the remaining elements. The elements do not need to be contiguous. The constraints...

Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

LeetCode 533: Lonely Pixel II

Problem Restatement We are given an m x n picture. Each cell is either: Character Meaning "B" Black pixel "W" White pixel We are also given an integer target . We need to count black pixels located at some position (r, c) such that both rules are true: Row r contains exactly target black pixels, and column c also contains exactly target black pixels. Every row that has a black...

Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

LeetCode 532: K-diff Pairs in an Array

Problem Restatement We are given an integer array nums and an integer k . We need to count the number of unique pairs (a, b) such that: abs(a - b) == k The pair must use two different indices in the array. The word unique is important. If the same value pair appears many times because of duplicates, we count it only once. The problem guarantees: 0 <= k So...

Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

LeetCode 531: Lonely Pixel I

Problem Restatement We are given an m x n picture. Each cell is either: Character Meaning "B" Black pixel "W" White pixel A black pixel is lonely if it is the only black pixel in its row and also the only black pixel in its column. We need to return the number of lonely black pixels. Input and Output Item Meaning Input A 2D character array picture Output The number...

Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

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 +...

Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

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"...

Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

LeetCode 419: Battleships in a Board

Problem Restatement We are given a 2D board containing: Character Meaning "X" Part of a battleship "." Empty water Battleships follow these rules: Battleships are placed either horizontally or vertically. Battleships cannot touch each other horizontally, vertically, or diagonally. A battleship consists of one or more consecutive "X" cells. Return the total number of battleships on the board. The official follow-up asks whether we can solve the problem in one...

Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 169: Majority Element

Problem Restatement We are given an integer array nums of size n . We need to return the majority element. The majority element is the value that appears more than: n // 2 times. The problem guarantees that a majority element always exists. LeetCode also asks whether we can solve it in linear time and constant extra space. Input and Output Item Meaning Input Integer array nums Output The majority...

Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

LeetCode 463: Island Perimeter

Problem Restatement We are given a rectangular grid. Each cell is either: 1 # land 0 # water The grid contains exactly one island. Land cells connect horizontally or vertically, not diagonally. The grid is surrounded by water, and the island has no lakes. We need to return the perimeter of the island. A land cell is a square with side length 1 . So one isolated land cell has...

Practice › LeetCode › LeetCode (archived) › LeetCode 03xx ›

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 233: Number of Digit One

Problem Restatement We are given a non-negative integer n . We need to count how many times the digit: 1 appears in all integers from: 0 to n inclusive. Example: n = 13 Numbers: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Digit 1 appears in: 1 10 11 11 12 13 Total count: 6 LeetCode examples include: Input: n = 13 Output: 6...

Practice › LeetCode › LeetCode (archived) › LeetCode 03xx ›

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 229: Majority Element II

Problem Restatement We are given an integer array nums of size n . We need to return all elements that appear more than: floor(n / 3) times. The answer can be returned in any order. LeetCode examples include nums = [3,2,3] , which returns [3] , and nums = [1,2] , which returns [1,2] . The constraints are 1 <= nums.length <= 5 * 10^4 and -10^9 <= nums[i] <=...

Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

LeetCode 454: 4Sum II

Problem Restatement We are given four integer arrays: nums1 nums2 nums3 nums4 All four arrays have the same length n . We need to count how many tuples (i, j, k, l) satisfy: 0 <= i, j, k, l < n and: nums1[i] + nums2[j] + nums3[k] + nums4[l] == 0 The task asks for the number of valid index tuples, not the tuples themselves. The official statement defines the...

Programming › Algorithm Cookbook › Chapter 5. Hashing and Maps ›

5.8 Counting Maps

5.8 Counting Maps Problem You need to count how many times each key appears in a dataset. A standard map stores one value per key. In counting problems, the value is a frequency. The task is to update counts efficiently while scanning input once. Solution Use a map from key to integer count. freq: Map<Key, Int> For each key, read the current count, then increment. count(items): freq = empty map...

Maths › 01. History and Biography ›

1.1 Tally Systems

Counting with marks The earliest way to count uses marks. A person makes one mark for each thing they want to count. If there are three animals, they draw three lines. If there are ten, they draw ten lines. Each mark stands for one object. You can see the total just by looking at the marks. This idea is simple but important. You match each object with one mark. This...

Maths › 01. History and Biography ›

1. Prehistoric and Early Counting

Chapter 1. Prehistoric and Early Counting Mathematics begins in practical activity. Early humans needed to track quantity, compare sizes, and organize their environment. These needs produced the first forms of counting and measurement. This chapter examines how these basic ideas appeared and how they shaped later mathematics. At this stage, mathematical thinking stays close to daily life. People count animals, measure land, track time, and divide resources. Each task requires...

Maths ›

05. Combinatorics

This volume studies finite and discrete structures, with emphasis on counting, arrangement, structure, and extremal behavior, and it maintains a constructive and algorithm-aware perspective throughout. Part I. Basic Counting Principles Chapter 1. Counting Foundations This chapter develops fundamental counting principles and techniques for establishing combinatorial identities. 1.1 Addition and multiplication principles 1.2 Bijective counting 1.3 Double counting 1.4 Inclusion-exclusion principle 1.5 Counting via symmetry Chapter 2. Permutations and Combinations This...

#counting

LeetCode 3438 - Find Valid Pair of Adjacent Digits in String

LeetCode 3890 - Integers With Multiple Sum of Two Cubes

LeetCode 3889 - Mirror Frequency Distance

LeetCode 3868 - Minimum Cost to Equalize Arrays Using Swaps

LeetCode 3866 - First Unique Even Element

LeetCode 3773 - Maximum Number of Equal Length Runs

LeetCode 3852 - Smallest Pair With Different Frequencies

LeetCode 3848 - Check Digitorial Permutation

LeetCode 3839 - Number of Prefix Connected Groups

LeetCode 3837 - Delayed Count of Equal Elements

LeetCode 3746 - Minimum String Length After Balanced Removals

LeetCode 3737 - Count Subarrays With Majority Element I

LeetCode 3720 - Lexicographically Smallest Permutation Greater Than Target

LeetCode 3712 - Sum of Elements With Frequency Divisible by K

LeetCode 3805 - Count Caesar Cipher Pairs

LeetCode 3692 - Majority Frequency Characters

LeetCode 3583 - Count Special Triplets

LeetCode 3679 - Minimum Discards to Balance Inventory

LeetCode 3715 - Sum of Perfect Square Ancestors

LeetCode 3664 - Two-Letter Card Game

LeetCode 3662 - Filter Characters by Frequency

LeetCode 3663 - Find The Least Frequent Digit

LeetCode 3636 - Threshold Majority Queries

LeetCode 3591 - Check if Any Element Has Prime Frequency

LeetCode 3545 - Minimum Deletions for At Most K Distinct Characters

LeetCode 3541 - Find Most Frequent Vowel and Consonant

LeetCode 3527 - Find the Most Common Response

LeetCode 3518 - Smallest Palindromic Rearrangement II

LeetCode 3467 - Transform Array by Parity

LeetCode 3442 - Maximum Difference Between Even and Odd Frequency I

LeetCode 3335 - Total Characters in String After Transformations I

LeetCode 1782 - Count Pairs Of Nodes

Numbers & Mathematics

LeetCode 1787 - Make the XOR of All Segments Equal to Zero

LeetCode 3267 - Count Almost Equal Pairs II

LeetCode 2268 - Minimum Number of Keypresses

LeetCode 2225 - Find Players With Zero or One Losses

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

LeetCode 3005 - Count Elements With Maximum Frequency

LeetCode 2453 - Destroy Sequential Targets

LeetCode 1994 - The Number of Good Subsets

LeetCode 2950 - Number of Divisible Substrings

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

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

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

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

LeetCode 2416 - Sum of Prefix Scores of Strings

LeetCode 2284 - Sender With Largest Word Count

LeetCode 2182 - Construct String With Repeat Limit

LeetCode 2529 - Maximum Count of Positive Integer and Negative Integer

LeetCode 3185 - Count Pairs That Form a Complete Day II

LeetCode 2006 - Count Number of Pairs With Absolute Difference K

LeetCode 2856 - Minimum Array Length After Pair Removals

LeetCode 2870 - Minimum Number of Operations to Make Array Empty

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

LeetCode 2423 - Remove Letter To Equalize Frequency

LeetCode 2514 - Count Anagrams

LeetCode 2287 - Rearrange Characters to Make Target String

LeetCode 2364 - Count Number of Bad Pairs

LeetCode 1876 - Substrings of Size Three with Distinct Characters

LeetCode 3279 - Maximum Total Area Occupied by Pistons

LeetCode 2048 - Next Greater Numerically Balanced Number

LeetCode 3039 - Apply Operations to Make String Empty

LeetCode 2341 - Maximum Number of Pairs in Array

LeetCode 3046 - Split the Array

LeetCode 2183 - Count Array Pairs Divisible by K

LeetCode 3223 - Minimum Length of String After Operations

LeetCode 2506 - Count Pairs Of Similar Strings

LeetCode 2404 - Most Frequent Even Element

LeetCode 1982 - Find Array Given Subset Sums

LeetCode 3085 - Minimum Deletions to Make String K-Special

LeetCode 2053 - Kth Distinct String in an Array

LeetCode 3128 - Right Triangles

LeetCode 2531 - Make Number of Distinct Characters Equal

LeetCode 2148 - Count Elements With Strictly Smaller and Greater Elements

LeetCode 2206 - Divide Array Into Equal Pairs

LeetCode 3265 - Count Almost Equal Pairs I

LeetCode 2960 - Count Tested Devices After Test Operations

LeetCode 2547 - Minimum Cost to Split an Array