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

LeetCode 823: Binary Trees With Factors

Problem Restatement We are given an array arr of unique integers. Every value is greater than 1 . We need to count how many binary trees can be built using values from arr . Each value may be used any number of times. For every non-leaf node: node.val == node.left.val * node.right.val Return the number of possible binary trees modulo: 10**9 + 7 The left and right child order matters,...

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LeetCode 819: Most Common Word

Problem Restatement We are given a string paragraph and a list of banned words. We need to return the most frequent word in paragraph that is not banned. Words are case-insensitive, so: "Bob" "bob" "BOB" all count as the same word. The answer must be returned in lowercase. Punctuation should be ignored. The paragraph may contain spaces and symbols such as: !?',;. The problem guarantees that at least one non-banned...

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LeetCode 940: Distinct Subsequences II

Problem Restatement We are given a string: s We need to count how many distinct non-empty subsequences exist. A subsequence is formed by deleting zero or more characters without changing the order of the remaining characters. For example, from: "abc" the subsequences include: "a" "b" "c" "ab" "ac" "bc" "abc" The empty subsequence does not count. Since the answer can become large, return it modulo: 10**9 + 7 The official...

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LeetCode 249: Group Shifted Strings

Problem Restatement We are given an array of lowercase strings. We need to group strings that belong to the same shifting sequence. A shift means changing every character to the next character in the alphabet: a -> b b -> c ... z -> a For example: "abc" -> "bcd" -> "cde" -> ... So these strings belong to the same group: "abc", "bcd", "xyz" because each one can be...

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LeetCode 246: Strobogrammatic Number

Problem Restatement We are given a string num that represents an integer. We need to return true if the number looks the same after being rotated 180 degrees. Otherwise, return false . Only some digits are valid after rotation: Digit Rotates To 0 0 1 1 6 9 8 8 9 6 Digits like 2 , 3 , 4 , 5 , and 7 cannot appear in a valid rotated...

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LeetCode 815: Bus Routes

Problem Restatement We are given a list of bus routes. Each routes[i] is the route of bus i . The bus repeats this route forever. For example: routes[0] = [1, 5, 7] means bus 0 travels like this forever: 1 -> 5 -> 7 -> 1 -> 5 -> 7 -> ... We start at bus stop source . We are not on any bus initially. We want to reach...

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LeetCode 792: Number of Matching Subsequences

Problem Restatement We are given a string s and an array of strings words . We need to return how many strings in words are subsequences of s . A subsequence is formed by deleting zero or more characters from a string without changing the order of the remaining characters. For example, "ace" is a subsequence of "abcde" . Input and Output Item Meaning Input A string s and a...

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LeetCode 244: Shortest Word Distance II

Problem Restatement We need to design a class called WordDistance . The class receives a list of words in the constructor. Then it must support a method called shortest(word1, word2) . This method returns the shortest index distance between word1 and word2 in the original list. The important detail is that shortest will be called many times with different pairs of words. So we should avoid scanning the whole word...

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LeetCode 791: Custom Sort String

Problem Restatement We are given two strings: order s All characters in order are unique. The string order defines a custom ordering of characters. We need to rearrange the characters of s so that characters appearing in order follow that same relative order. Characters in s that do not appear in order may be placed anywhere in the result. Return any valid permutation of s . Input and Output Item...

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LeetCode 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...

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LeetCode 835: Image Overlap

Problem Restatement We are given two binary square matrices img1 and img2 , both of size n x n . Each cell is either: Value Meaning 0 Empty pixel 1 Filled pixel We may translate one image by sliding it left, right, up, or down by any number of units. Translation does not include rotation. After translation, some 1 cells may move outside the matrix border and disappear. The overlap...

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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...

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LeetCode 811: Subdomain Visit Count

Problem Restatement We are given a list of count-paired domains. Each item has this format: "count domain" For example: "9001 discuss.leetcode.com" This means discuss.leetcode.com was visited 9001 times. A visit to a domain also counts as a visit to all of its parent subdomains: discuss.leetcode.com leetcode.com com Return a list of count-paired domains showing the total visit count for every domain and subdomain. The result may be returned in any...

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LeetCode 692: Top K Frequent Words

Problem Restatement We are given a list of words and an integer k . We need to return the k most frequent words. The ordering rules are: Rule Meaning Higher frequency first More common words come earlier Lexicographical tie-break If two words have the same frequency, the smaller alphabetical word comes first The official statement defines lexicographical order using normal alphabetical string comparison. Input and Output Item Meaning Input words...

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LeetCode 690: Employee Importance

Problem Restatement We are given a list of employees. Each employee has: Field Meaning id Unique employee ID importance Integer importance value subordinates List of direct subordinate IDs Given an employee ID, return the total importance of that employee and all their direct and indirect subordinates. The official statement asks for the sum of the importance values for the given employee, plus every employee under them in the hierarchy. Direct...

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LeetCode 642: Design Search Autocomplete System

Problem Restatement We need to design an autocomplete system. The system starts with historical sentences and their usage counts. For every typed character except # , return up to three historical sentences that start with the current prefix. The result must be sorted by: Priority Rule First Higher frequency first Second ASCII order if frequencies are equal When the user types # , the current sentence is finished. The system...

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LeetCode 833: Find And Replace in String

Problem Restatement We are given a string s and three arrays: Array Meaning indices Starting positions where replacement may happen sources Source strings that must match at those positions targets Replacement strings For each operation i , we check whether sources[i] appears in the original string s starting at index indices[i] . If it matches, we replace that source substring with targets[i] . If it does not match, we do...

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LeetCode 666: Path Sum IV

Problem Restatement We are given an array nums . Each integer in nums has exactly three digits and represents one node in a binary tree. For a number DPV : Digit Meaning D Depth of the node P Position of the node within that depth V Value of the node Depth and position are 1-based. The tree depth is less than 5 , so valid depths are from 1 to...

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LeetCode 781: Rabbits in Forest

Problem Restatement We are given an array answers . Each value answers[i] is the answer from one rabbit. If a rabbit says x , it means: There are x other rabbits with the same color as me. So that rabbit belongs to a color group of size: x + 1 We need to return the minimum possible number of rabbits in the forest. Some rabbits in the forest may not...

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

LeetCode 930: Binary Subarrays With Sum

Problem Restatement We are given a binary array nums and an integer goal . A binary array means every value is either: 0 or: 1 We need to count how many non-empty contiguous subarrays have sum exactly equal to goal . A subarray must be continuous. We cannot skip elements. The official statement asks for the number of non-empty subarrays whose sum is goal , where nums is binary. Example...

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

LeetCode 525: Contiguous Array

Problem Restatement We are given a binary array nums . Each element is either: 0 or 1 We need to return the maximum length of a contiguous subarray that contains an equal number of 0 s and 1 s. The official problem asks for the maximum length of a contiguous subarray with an equal number of 0 and 1 . Input and Output Item Meaning Input A binary array nums...

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LeetCode 631: Design Excel Sum Formula

Problem Restatement We need to design a simplified Excel spreadsheet. The spreadsheet has: Item Meaning height Number of rows width Last column letter, from A to Z Initial value Every cell starts as 0 We need to support three operations: Operation Meaning set(row, column, val) Set a cell to a fixed integer value get(row, column) Return the current value of a cell sum(row, column, numbers) Set a cell to a...

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

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

Problem Restatement We are given a string s . For any string t , define countUniqueChars(t) as the number of characters that appear exactly once in t . We need to return the sum of countUniqueChars(t) over every substring t of s . The answer is guaranteed to fit in a 32-bit integer. Input and Output Item Meaning Input A string s Output Sum of unique-character counts over all substrings...

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LeetCode 523: Continuous Subarray Sum

Problem Restatement We are given: nums k We need to determine whether there exists a continuous subarray of length at least 2 whose sum is a multiple of k . That means we need some subarray sum satisfying: $$ subarray_sum = n \cdot k $$ for some integer n . Equivalently: $$ subarray_sum \bmod k = 0 $$ The official problem asks whether such a subarray exists, with the subarray...

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LeetCode 659: Split Array into Consecutive Subsequences

Problem Restatement We are given an integer array nums . The array is sorted in non-decreasing order. We need to decide whether we can split all numbers into one or more subsequences such that: Rule Meaning Consecutive Each number is exactly 1 larger than the previous number Minimum length Every subsequence has length at least 3 Use all numbers Every element in nums must be used exactly once Return True...

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LeetCode 599: Minimum Index Sum of Two Lists

Problem Restatement We are given two string arrays, list1 and list2 . A common string is a string that appears in both arrays. For each common string, compute its index sum: index in list1 + index in list2 We need to return all common strings with the smallest index sum. The answer may be returned in any order. The problem guarantees that both lists have unique strings and that there...

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LeetCode 519: Random Flip Matrix

Problem Restatement We are given a binary matrix with: m rows n columns Initially, every cell contains: 0 We need to design a data structure supporting two operations. Operations flip() Randomly choose one remaining cell containing 0 . Change it to: 1 Return its coordinates. Every remaining zero cell must have equal probability of being chosen. reset() Set all cells back to: 0 The official problem asks us to randomly...

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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...

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LeetCode 567: Permutation in String

Problem Restatement We are given two strings s1 and s2 . Return true if s2 contains a permutation of s1 as a substring. Otherwise, return false . A permutation means the same characters with the same frequencies, but possibly in a different order. So the problem is asking: Does s2 contain any contiguous substring with exactly the same character counts as s1 ? The official constraints are 1 <= s1.length,...

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LeetCode 508: Most Frequent Subtree Sum

Problem Restatement We are given the root of a binary tree. For every node, define its subtree sum as the sum of all values in the subtree rooted at that node. This includes the node itself and all of its descendants. We need to return the subtree sum value or values that appear most frequently. If several subtree sums have the same highest frequency, return all of them in any...

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LeetCode 506: Relative Ranks

Problem Restatement We are given an integer array score . score[i] is the score of the i th athlete. All scores are unique. Athletes are ranked from highest score to lowest score: Place Rank string 1st "Gold Medal" 2nd "Silver Medal" 3rd "Bronze Medal" 4th and later The placement number as a string We need to return an array answer where: answer[i] is the rank of the i th athlete...

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LeetCode 588: Design In-Memory File System

Problem Restatement We need to design an in-memory file system. The file system must support four operations: Operation Behavior ls(path) If path is a file, return only that file name. If path is a directory, return its direct children in lexicographic order. mkdir(path) Create the directory path. If middle directories do not exist, create them too. addContentToFile(filePath, content) Create the file if missing, otherwise append content to the existing file....

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

LeetCode 677: Map Sum Pairs

Problem Restatement We need to design a data structure called MapSum . It supports two operations: Operation Meaning insert(key, val) Store the value val for the string key sum(prefix) Return the total value of all keys that start with prefix If a key already exists, the new value replaces the old value. It does not add to the old value. For example: insert("apple", 3) sum("ap") # 3 insert("app", 2) sum("ap")...

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LeetCode 726: Number of Atoms

Problem Restatement We are given a valid chemical formula string. The formula contains atom names, numbers, and parentheses. We need to count how many times each atom appears, then return the result as a string sorted by atom name. An atom name starts with one uppercase letter. It may be followed by lowercase letters. Examples: "H" "Mg" "Ca" A number after an atom means the atom appears that many times....

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LeetCode 676: Implement Magic Dictionary

Problem Restatement We need to design a data structure called MagicDictionary . It supports two operations: Operation Meaning buildDict(dictionary) Store a list of distinct words search(searchWord) Return whether searchWord can become a dictionary word by changing exactly one character The important rule is exactly one character must be changed. Zero changes do not count. Two or more changes do not count. The modified word must exist in the dictionary. For...

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LeetCode 652: Find Duplicate Subtrees

Problem Restatement We are given the root of a binary tree. We need to find all duplicate subtrees. Two subtrees are duplicate when they have: Requirement Meaning Same structure The nodes are arranged in the same shape Same values Matching nodes have equal values For each kind of duplicate subtree, we only return one root node. For example, if the same subtree appears three times, we still add only one...

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LeetCode 560: Subarray Sum Equals K

Problem Restatement We are given an integer array nums and an integer k . Return the total number of non-empty contiguous subarrays whose sum is exactly equal to k . A subarray must use consecutive elements. The problem constraints are 1 <= nums.length <= 2 * 10^4 , -1000 <= nums[i] <= 1000 , and -10^7 <= k <= 10^7 . The array may contain positive numbers, negative numbers, and...

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LeetCode 582: Kill Process

Problem Restatement We are given a list of process IDs and their parent process IDs. Each process forms part of a process tree. If a process is killed, then all of its child processes are also killed recursively. Given a process ID called kill , return all process IDs that will be terminated. The result may be returned in any order. The official problem defines: pid[i] = process ID ppid[i]...

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LeetCode 554: Brick Wall

Problem Restatement We are given a rectangular brick wall. The wall has several rows. Each row contains bricks of different widths, but every row has the same total width. We need to draw one vertical line from the top of the wall to the bottom. If the line passes through the middle of a brick, it crosses that brick. If the line passes exactly through the edge between two bricks,...

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LeetCode 465: Optimal Account Balancing

Problem Restatement We are given a list of transactions. Each transaction has the form: [from_person, to_person, amount] This means from_person gave amount dollars to to_person . After all original transactions, some people may owe money and some people may be owed money. We need to return the minimum number of transactions required to settle all debts. The official problem defines each transaction as [from_i, to_i, amount_i] and asks for the...

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

LeetCode 220: Contains Duplicate III

Problem Restatement We are given an integer array nums and two integers: indexDiff valueDiff We need to return True if there are two different indices i and j such that: i != j abs(i - j) <= indexDiff abs(nums[i] - nums[j]) <= valueDiff Otherwise, return False . The official problem asks for a pair of distinct indices whose index distance is at most indexDiff and whose value difference is at...

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LeetCode 219: Contains Duplicate II

Problem Restatement We are given an integer array nums and an integer k . We need to return True if there are two different indices i and j such that: nums[i] == nums[j] and: abs(i - j) <= k Otherwise, return False . The official statement asks whether two distinct indices have equal values and index distance at most k . Constraints include 1 <= nums.length <= 10^5 , -10^9...

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LeetCode 149: Max Points on a Line

Problem Restatement We are given a list of 2D points: points[i] = [xi, yi] Return the maximum number of points that lie on the same straight line. A line may be horizontal, vertical, diagonal, or any slope. LeetCode constrains the number of points to at most 300 , which allows an O(n^2) solution. Examples Example 1: points = [[1,1],[2,2],[3,3]] All three points lie on: y = x Output: 3 Example...

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LeetCode 146: LRU Cache

Problem Restatement Design a data structure that supports: Operation Meaning get(key) Return the value if the key exists, otherwise -1 put(key, value) Insert or update the key-value pair The cache has a fixed capacity. When the cache exceeds capacity, remove the least recently used item. Both operations must run in: O(1) average time. LeetCode defines the least recently used item as the entry that has not been accessed or updated...

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LeetCode 138: Copy List with Random Pointer

Problem Restatement We are given the head of a linked list. Each node contains: Field Meaning val Integer value next Pointer to the next node random Pointer to any node in the list or None We need to create a deep copy of the entire list. The copied list must contain entirely new nodes. Every copied node should preserve: The same val The same next structure The same random structure...

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LeetCode 460: LFU Cache

Problem Restatement We need to design a Least Frequently Used cache. The cache supports two operations: get(key) put(key, value) The cache has a fixed capacity . If the cache is full and we need to insert a new key, we remove the least frequently used key. Each key has a use counter. A key's frequency increases when: get(key) is called on it. put(key, value) is called on an existing key....

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LeetCode 133: Clone Graph

Problem Restatement We are given a reference to a node in a connected undirected graph. Each node has: Field Meaning val Integer value of the node neighbors List of neighboring node references We need to return a deep copy of the graph. A deep copy means: Every original node gets a new node object. Every edge is copied between the new node objects. No cloned node should point back to...

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LeetCode 106: Construct Binary Tree from Inorder and Postorder Traversal

Problem Restatement We are given two integer arrays: inorder postorder Both arrays describe the same binary tree. inorder is the inorder traversal of the tree. postorder is the postorder traversal of the same tree. We need to construct and return the original binary tree. The values are unique, and both arrays are guaranteed to describe the same tree. The official constraints also say each value in postorder appears in inorder...

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LeetCode 105: Construct Binary Tree from Preorder and Inorder Traversal

Problem Restatement We are given two integer arrays: preorder inorder Both arrays describe the same binary tree. preorder is the preorder traversal of the tree. inorder is the inorder traversal of the same tree. We need to construct and return the original binary tree. The official problem states that preorder and inorder are traversals of the same tree. The key traversal rules are: preorder: root -> left -> right inorder:...

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LeetCode 205: Isomorphic Strings

Problem Restatement We are given two strings: s t We need to determine whether the two strings are isomorphic. Two strings are isomorphic when characters from s can be replaced to form t while preserving structure. Rules: Every character in s must map to exactly one character in t . Different characters in s cannot map to the same character in t . A character may map to itself. The...

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LeetCode 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...

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LeetCode 451: Sort Characters By Frequency

Problem Restatement We are given a string s . We need to rearrange its characters so that characters with higher frequency appear before characters with lower frequency. The frequency of a character means how many times it appears in the string. If two characters have the same frequency, their relative order does not matter. Any valid order is accepted. For example: s = "tree" The character frequencies are: Character Frequency...

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LeetCode 76: Minimum Window Substring

Problem Restatement We are given two strings: s: str t: str We need to return the shortest substring of s that contains every character from t . Characters must be counted with duplicates. So if: t = "AABC" then a valid window must contain: A: 2 B: 1 C: 1 If no substring of s can contain all characters from t , return: "" The official problem states that s...

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LeetCode 1: Two Sum

Problem Restatement We are given an array of integers called nums and an integer called target . We need to find two different positions in the array such that the two numbers at those positions add up to target . Return the two indices. The same element cannot be used twice. That means if nums[i] is used as the first number, we need another index j , where i !=...

Wiki › Algorithms › 02. Data Structures › 1. Core Data Structures and Operations › 1.1 Arrays and Dynamic Arrays ›

Sparse Array

Sparse Array A sparse array represents a large logical array by storing only positions whose values differ from a default value. Most entries are implicit. You use it when the index space is large but only a small number of positions contain meaningful data. Problem Given a logical array $A$ of length $n$, support reading and writing index $i$ where: $$ 0 \le i < n $$ Most values are...

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LeetCode 823: Binary Trees With Factors

LeetCode 819: Most Common Word

LeetCode 940: Distinct Subsequences II

LeetCode 249: Group Shifted Strings

LeetCode 246: Strobogrammatic Number

LeetCode 815: Bus Routes

LeetCode 792: Number of Matching Subsequences

LeetCode 244: Shortest Word Distance II

LeetCode 791: Custom Sort String

LeetCode 621: Task Scheduler

LeetCode 835: Image Overlap

LeetCode 697: Degree of an Array

LeetCode 811: Subdomain Visit Count

LeetCode 692: Top K Frequent Words

LeetCode 690: Employee Importance

LeetCode 642: Design Search Autocomplete System

LeetCode 833: Find And Replace in String

LeetCode 666: Path Sum IV

LeetCode 781: Rabbits in Forest

LeetCode 930: Binary Subarrays With Sum

LeetCode 525: Contiguous Array

LeetCode 631: Design Excel Sum Formula

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

LeetCode 523: Continuous Subarray Sum

LeetCode 659: Split Array into Consecutive Subsequences

LeetCode 599: Minimum Index Sum of Two Lists

LeetCode 519: Random Flip Matrix

LeetCode 594: Longest Harmonious Subsequence

LeetCode 567: Permutation in String

LeetCode 508: Most Frequent Subtree Sum

LeetCode 506: Relative Ranks

LeetCode 588: Design In-Memory File System

LeetCode 677: Map Sum Pairs

LeetCode 726: Number of Atoms

LeetCode 676: Implement Magic Dictionary

LeetCode 652: Find Duplicate Subtrees

LeetCode 560: Subarray Sum Equals K

LeetCode 582: Kill Process

LeetCode 554: Brick Wall

LeetCode 465: Optimal Account Balancing

LeetCode 220: Contains Duplicate III

LeetCode 219: Contains Duplicate II

LeetCode 149: Max Points on a Line

LeetCode 146: LRU Cache

LeetCode 138: Copy List with Random Pointer

LeetCode 460: LFU Cache

LeetCode 133: Clone Graph

LeetCode 106: Construct Binary Tree from Inorder and Postorder Traversal

LeetCode 105: Construct Binary Tree from Preorder and Inorder Traversal

LeetCode 205: Isomorphic Strings

LeetCode 454: 4Sum II

LeetCode 451: Sort Characters By Frequency

LeetCode 76: Minimum Window Substring

LeetCode 1: Two Sum

Sparse Array