LeetCode 00xx

LeetCode problems 1–99

99 items

LeetCode 00xx

# Problem Difficulty Time Notes
1 Two Sum 🟢 Easy 4m The problem gives us an integer array called nums and another integ…
2 Add Two Numbers 🟡 Medium 2m 35s The problem gives us two non empty singly linked lists. Each linked…
3 Longest Substring Without Repeating Characters 🟡 Medium 1m 9s The problem gives a string s and asks for the length of the longest…
4 Median of Two Sorted Arrays 🔴 Hard 1m 25s The problem gives two individually sorted integer arrays, nums1 and…
5 Longest Palindromic Substring 🟡 Medium 3m 13s The problem asks us to find the longest substring of a given string…
6 Zigzag Conversion 🟡 Medium 1m 31s The problem asks us to transform a string into a zigzag pattern acr…
7 Reverse Integer 🟡 Medium 1m 15s The problem asks us to reverse the digits of a signed 32-bit intege…
8 String to Integer (atoi) 🟡 Medium 1m 52s The problem asks us to implement a simplified version of the C/C++ …
9 Palindrome Number 🟢 Easy 1m 47s The problem asks us to determine whether a given integer reads the …
10 Regular Expression Matching 🔴 Hard 6m 5s LeetCode 10, Regular Expression Matching, asks us to determine whet…
11 Container With Most Water 🟡 Medium 1m 11s The problem gives an array called height, where each element repres…
12 Integer to Roman 🟡 Medium 1m 27s The problem asks us to convert a positive integer into its Roman nu…
13 Roman to Integer 🟢 Easy 1m 11s The problem gives a Roman numeral string and asks us to convert it …
14 Longest Common Prefix 🟢 Easy 1m 24s The problem gives an array of strings and asks us to find the longe…
15 3Sum 🟡 Medium 1m 43s The problem gives us an integer array nums, and we need to find eve…
16 3Sum Closest 🟡 Medium 1m 12s The problem gives us an integer array nums and a target integer tar…
17 Letter Combinations of a Phone Number 🟡 Medium 3m 42s The problem gives a string of digits where each digit is between 2 …
18 4Sum 🟡 Medium 2m 28s We are given an integer array nums and an integer target. The goal …
19 Remove Nth Node From End of List 🟡 Medium 1m 7s This problem asks us to remove the nth node counted from the end of…
20 Valid Parentheses 🟢 Easy 1m 6s The problem gives a string containing only six possible characters:…
21 Merge Two Sorted Lists 🟢 Easy 2m 51s This problem asks us to merge two already sorted singly linked list…
22 Generate Parentheses 🟡 Medium 1m 18s The problem asks us to generate every possible valid combination of…
23 Merge k Sorted Lists 🔴 Hard 1m 14s The problem gives us an array of k sorted linked lists. Each linked…
24 Swap Nodes in Pairs 🟡 Medium 2m 58s The problem gives us the head of a singly linked list and asks us t…
25 Reverse Nodes in k-Group 🔴 Hard 1m 46s This problem asks us to reverse a singly linked list in groups of s…
26 Remove Duplicates from Sorted Array 🟢 Easy 1m 55s The problem gives us a sorted integer array nums in non-decreasing …
27 Remove Element 🟢 Easy 1m 3s The problem gives us an integer array nums and a target value val. …
28 Find the Index of the First Occurrence in a String 🟢 Easy 5m 54s This problem asks us to locate the first occurrence of one string i…
29 Divide Two Integers 🟡 Medium 1m 29s The problem asks us to implement integer division without using the…
30 Substring with Concatenation of All Words 🔴 Hard 1m 1s The problem asks us to find all starting indices in a string s wher…
31 Next Permutation 🟡 Medium 1m 8s The problem asks us to modify an integer array so that it becomes t…
32 Longest Valid Parentheses 🔴 Hard 1m 6s The problem asks us to find the length of the longest contiguous su…
33 Search in Rotated Sorted Array 🟡 Medium 1m 14s The problem gives us an array that was originally sorted in ascendi…
34 Find First and Last Position of Element in Sorted Array 🟡 Medium 1m 9s The problem gives us a sorted integer array nums and a target value…
35 Search Insert Position 🟢 Easy 1m 5s The problem gives us a sorted array of distinct integers and a targ…
36 Valid Sudoku 🟡 Medium 1m 21s This problem asks us to validate whether a partially filled Sudoku …
37 Sudoku Solver 🔴 Hard 1m 30s The problem asks us to build a complete Sudoku solver. We are given…
38 Count and Say 🟡 Medium 1m 10s The problem asks us to generate the nth term of the "count-and-say"…
39 Combination Sum 🟡 Medium 1m 3s This problem asks us to generate every possible unique combination …
40 Combination Sum II 🟡 Medium 1m 28s This problem asks us to find all unique combinations of numbers fro…
41 First Missing Positive 🔴 Hard 1m 24s The problem asks us to find the smallest positive integer that does…
42 Trapping Rain Water 🔴 Hard 1m 25s The problem gives an array called height, where each element repres…
43 Multiply Strings 🟡 Medium 1m 33s The problem asks us to multiply two non-negative integers where eac…
44 Wildcard Matching 🔴 Hard 1m 36s This problem asks us to determine whether an entire input string s …
45 Jump Game II 🟡 Medium 1m 5s The problem gives an array nums where each value represents the max…
46 Permutations 🟡 Medium 1m 15s The problem gives us an array of distinct integers called nums, and…
47 Permutations II 🟡 Medium 1m 44s The problem asks us to generate every possible permutation of the g…
48 Rotate Image 🟡 Medium 1m 26s The problem gives us an n x n square matrix that represents an imag…
49 Group Anagrams 🟡 Medium 1m 38s The problem asks us to group together all strings that are anagrams…
50 Pow(x, n) 🟡 Medium 1m 26s The problem asks us to implement exponentiation manually, specifica…
51 N-Queens 🔴 Hard 3m 41s The N-Queens problem asks us to place n queens on an n x n chessboa…
52 N-Queens II 🔴 Hard 1m 7s The n-queens puzzle asks us to place n queens on an n x n chessboar…
53 Maximum Subarray 🟡 Medium 1m 2s The problem gives an integer array nums and asks us to find a conti…
54 Spiral Matrix 🟡 Medium 1m 8s The problem gives an m x n matrix and asks us to return all element…
55 Jump Game 🟡 Medium 1m 44s The problem gives us an integer array nums, where each value repres…
56 Merge Intervals 🟡 Medium 1m 30s The problem gives an array of intervals, where each interval is rep…
57 Insert Interval 🟡 Medium 1m 15s The problem gives us a list of intervals that are already sorted by…
58 Length of Last Word 🟢 Easy 1m 14s The problem gives us a string s that contains English letters and s…
59 Spiral Matrix II 🟡 Medium 1m 34s The problem asks us to generate an n x n matrix and fill it with nu…
60 Permutation Sequence 🔴 Hard 5m 16s The problem gives us the numbers from 1 to n, and asks us to find t…
61 Rotate List 🟡 Medium 1m 11s This problem asks us to rotate a singly linked list to the right by…
62 Unique Paths 🟡 Medium 1m 3s The problem describes a robot moving on a rectangular m x n grid. T…
63 Unique Paths II 🟡 Medium 1m 22s This problem asks us to count how many different valid paths exist …
64 Minimum Path Sum 🟡 Medium 2m 24s The problem gives us a two dimensional grid of size m x n, where ea…
65 Valid Number 🔴 Hard 2m 46s This is a comprehensive, multi section technical guide that will be…
66 Plus One 🟢 Easy 2m 21s The problem gives us a non-empty array of digits that together repr…
67 Add Binary 🟢 Easy 2m 25s You are given two strings, a and b, where each string represents a …
68 Text Justification 🔴 Hard 1m 23s This problem asks us to simulate how text is formatted in a text ed…
69 Sqrt(x) 🟢 Easy 57s The problem asks us to compute the integer square root of a non neg…
70 Climbing Stairs 🟢 Easy 1m 8s The problem describes a staircase with n total steps. You start at …
71 Simplify Path 🟡 Medium 1m 1s The problem gives us an absolute Unix-style file path and asks us t…
72 Edit Distance 🟡 Medium 1m 37s The problem asks us to compute the minimum number of edit operation…
73 Set Matrix Zeroes 🟡 Medium 1m 27s The problem gives us a two dimensional matrix of integers with m ro…
74 Search a 2D Matrix 🟡 Medium 1m 36s This problem gives us a two dimensional matrix with two very import…
75 Sort Colors 🟡 Medium 6m 34s This problem asks us to sort an array that contains only three dist…
76 Minimum Window Substring 🔴 Hard 1m 30s The problem asks us to find the smallest contiguous substring insid…
77 Combinations 🟡 Medium 1m 3s This problem asks us to generate all possible combinations of size …
78 Subsets 🟡 Medium 2m 23s This problem asks us to generate every possible subset of a given a…
79 Word Search 🟡 Medium 2m 21s In this problem, we are given a two dimensional grid of characters …
80 Remove Duplicates from Sorted Array II 🟡 Medium 4m 7s This problem asks us to modify a sorted integer array in-place so t…
81 Search in Rotated Sorted Array II 🟡 Medium 1m 10s This problem asks us to determine whether a target value exists ins…
82 Remove Duplicates from Sorted List II 🟡 Medium 1m 11s This problem asks us to process a sorted singly linked list and rem…
83 Remove Duplicates from Sorted List 🟢 Easy 4m 18s This problem asks us to remove duplicate values from a sorted singl…
84 Largest Rectangle in Histogram 🔴 Hard 1m 12s This problem asks us to find the largest rectangular area that can …
85 Maximal Rectangle 🔴 Hard 1m 23s The problem asks us to find the area of the largest rectangle conta…
86 Partition List 🟡 Medium 42s It looks like you want the full detailed solution guide, but the sp…
87 Scramble String 🔴 Hard 2m 21s This problem asks us to determine whether one string can be transfo…
88 Merge Sorted Array 🟢 Easy 1m 33s This problem gives us two arrays, nums1 and nums2, where both array…
89 Gray Code 🟡 Medium 3m 16s The problem asks us to generate a valid Gray code sequence for a gi…
90 Subsets II 🟡 Medium 1m 22s The problem asks us to generate every possible subset of a given in…
91 Decode Ways 🟡 Medium 1m 21s This problem asks us to determine how many different valid ways a n…
92 Reverse Linked List II 🟡 Medium 1m 9s This problem asks us to reverse only a specific portion of a singly…
93 Restore IP Addresses 🟡 Medium 1m 19s The problem asks us to take a string containing only digits and det…
94 Binary Tree Inorder Traversal 🟢 Easy 1m 17s This problem asks us to perform an inorder traversal on a binary tr…
95 Unique Binary Search Trees II 🟡 Medium 36s The problem asks us to generate all structurally unique binary sear…
96 Unique Binary Search Trees 🟡 Medium 1m 8s This problem asks us to determine how many structurally different B…
97 Interleaving String 🟡 Medium 1m 27s This problem asks us to determine whether a string s3 can be formed…
98 Validate Binary Search Tree 🟡 Medium 1m 10s This problem asks us to determine whether a given binary tree satis…
99 Recover Binary Search Tree 🟡 Medium 1m 12s The problem gives us the root of a binary search tree, but exactly …
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LeetCode 1 - Two Sum

The problem gives us an integer array called nums and another integer called target. Our task is to find two different elements in the array whose sum equals target, then return their indices.
leetcode Easy array hash-table May 27, 2026
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LeetCode 2 - Add Two Numbers

The problem gives us two non empty singly linked lists. Each linked list represents a non negative integer, but the digits are stored in reverse order. That means the head node contains the least significant digit.
leetcode Medium linked-list math recursion May 27, 2026
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LeetCode 3 - Longest Substring Without Repeating Characters

The problem gives a string s and asks for the length of the longest substring that contains no repeated characters. A substring is a continuous section of the string. This detail matters because characters must remain adjacent.
leetcode Medium hash-table string sliding-window May 27, 2026
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LeetCode 4 - Median of Two Sorted Arrays

The problem gives two individually sorted integer arrays, nums1 and nums2, with lengths m and n. The task is to compute the median of the combined sorted sequence formed by merging both arrays. The median is the middle value of a sorted sequence.
leetcode Hard array binary-search divide-and-conquer May 27, 2026
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LeetCode 5 - Longest Palindromic Substring

The problem asks us to find the longest substring of a given string that is also a palindrome. A palindrome is a sequence that reads the same forward and backward. The substring must be contiguous, which means the characters must appear next to each other in the original string.
leetcode Medium two-pointers string dynamic-programming May 27, 2026
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LeetCode 6 - Zigzag Conversion

The problem asks us to transform a string into a zigzag pattern across a fixed number of rows, then read the characters row by row to produce the final result.
leetcode Medium string May 27, 2026
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LeetCode 7 - Reverse Integer

The problem asks us to reverse the digits of a signed 32-bit integer. Given an integer x, we must return a new integer whose digits appear in reverse order while preserving the sign. For example, if the input is 123, reversing the digits produces 321.
leetcode Medium math May 27, 2026
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LeetCode 8 - String to Integer (atoi)

The problem asks us to implement a simplified version of the C/C++ atoi function, which converts a string into a 32-bit signed integer. The conversion process is strict and follows several rules in a specific order.
leetcode Medium string May 27, 2026
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LeetCode 9 - Palindrome Number

The problem asks us to determine whether a given integer reads the same forward and backward. Such numbers are called palindromes. For example, the number 121 is a palindrome because reversing its digits still produces 121.
leetcode Easy math May 27, 2026
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LeetCode 10 - Regular Expression Matching

LeetCode 10, Regular Expression Matching, asks us to determine whether an entire input string s matches a pattern p. The pattern supports two special regular expression characters: - .
leetcode Hard string dynamic-programming recursion May 28, 2026
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LeetCode 11 - Container With Most Water

The problem gives an array called height, where each element represents the height of a vertical line drawn on a coordinate plane. The line at index i starts at (i, 0) and ends at (i, height[i]).
leetcode Medium array two-pointers greedy May 27, 2026
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LeetCode 12 - Integer to Roman

The problem asks us to convert a positive integer into its Roman numeral representation. Roman numerals use combinations of specific symbols to represent values, and the rules for combining those symbols are strict.
leetcode Medium hash-table math string May 28, 2026
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LeetCode 13 - Roman to Integer

The problem gives a Roman numeral string and asks us to convert it into its corresponding integer value. Roman numerals use seven symbols: Symbol Value --- --- I 1 V 5 X 10 L 50 C 100 D 500 M 1000 Most Roman numerals follow a simple additive rule.
leetcode Easy hash-table math string May 27, 2026
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LeetCode 14 - Longest Common Prefix

The problem gives an array of strings and asks us to find the longest prefix shared by every string in the array. A prefix is a sequence of characters that appears at the beginning of a string.
leetcode Easy array string trie May 28, 2026
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LeetCode 15 - 3Sum

The problem gives us an integer array nums, and we need to find every unique triplet of numbers whose sum is exactly 0. A triplet consists of three different indices: - i != j - i != k - j != k The values themselves may be equal, but the indices must be different.
leetcode Medium array two-pointers sorting May 28, 2026
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LeetCode 16 - 3Sum Closest

The problem gives us an integer array nums and a target integer target. We must select exactly three elements from the array, using distinct indices, and compute their sum. Among all possible three-number sums, we want the one whose value is closest to the target.
leetcode Medium array two-pointers sorting May 28, 2026
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LeetCode 17 - Letter Combinations of a Phone Number

The problem gives a string of digits where each digit is between 2 and 9. Each digit corresponds to a set of letters on a traditional phone keypad: - 2 - "abc" - 3 - "def" - 4 - "ghi" - 5 - "jkl" - 6 - "mno" - 7 - "pqrs" - 8 - "tuv" - 9 - "wxyz" We must generate every possible…
leetcode Medium hash-table string backtracking May 28, 2026
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LeetCode 18 - 4Sum

We are given an integer array nums and an integer target. The goal is to find every unique quadruplet of numbers in the array whose sum equals the target value. A quadruplet consists of four elements: where all four indices are distinct.
leetcode Medium array two-pointers sorting May 28, 2026
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LeetCode 19 - Remove Nth Node From End of List

This problem asks us to remove the nth node counted from the end of a singly linked list and return the modified list. A singly linked list is a sequence of nodes where each node contains a value and a pointer to the next node.
leetcode Medium linked-list two-pointers May 28, 2026
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LeetCode 20 - Valid Parentheses

The problem gives a string containing only six possible characters: (, ), {, }, [ and ]. Each opening bracket must eventually be matched with the correct closing bracket, and the order of matching matters.
leetcode Easy string stack May 28, 2026
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LeetCode 21 - Merge Two Sorted Lists

This problem asks us to merge two already sorted singly linked lists into one new sorted linked list. The important detail is that we are not creating entirely new nodes for the merged result. Instead, we reuse the existing nodes by reconnecting their next pointers.
leetcode Easy linked-list recursion May 28, 2026
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LeetCode 22 - Generate Parentheses

The problem asks us to generate every possible valid combination of n pairs of parentheses. A valid parentheses string is one where every opening parenthesis ( has a matching closing parenthesis ), and the parentheses are properly nested.
leetcode Medium string dynamic-programming backtracking May 28, 2026
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LeetCode 23 - Merge k Sorted Lists

The problem gives us an array of k sorted linked lists. Each linked list is already sorted in ascending order, and our task is to combine all of them into one single sorted linked list. A linked list node contains a value and a pointer to the next node.
leetcode Hard linked-list divide-and-conquer heap-(priority-queue) merge-sort May 28, 2026
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LeetCode 24 - Swap Nodes in Pairs

The problem gives us the head of a singly linked list and asks us to swap every pair of adjacent nodes. The important detail is that we are not allowed to modify the values stored inside the nodes. We must physically rearrange the node connections by changing pointers.
leetcode Medium linked-list recursion May 28, 2026
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LeetCode 25 - Reverse Nodes in k-Group

This problem asks us to reverse a singly linked list in groups of size k. Instead of reversing the entire list at once, we only reverse consecutive chunks containing exactly k nodes.
leetcode Hard linked-list recursion May 28, 2026
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LeetCode 26 - Remove Duplicates from Sorted Array

The problem gives us a sorted integer array nums in non-decreasing order. Because the array is already sorted, any duplicate values will always appear next to each other.
leetcode Easy array two-pointers May 28, 2026
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LeetCode 27 - Remove Element

The problem gives us an integer array nums and a target value val. Our task is to remove every occurrence of val from the array, but we must do it in-place. This means we are not supposed to create another array and return it. Instead, we modify the original array directly.
leetcode Easy array two-pointers May 28, 2026
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LeetCode 28 - Find the Index of the First Occurrence in a String

This problem asks us to locate the first occurrence of one string inside another string. The string we are searching for is called needle, and the larger string we search inside is called haystack.
leetcode Easy two-pointers string string-matching May 28, 2026
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LeetCode 29 - Divide Two Integers

The problem asks us to implement integer division without using the standard arithmetic operators for multiplication, division, or modulo. We are given two integers, dividend and divisor, and we must compute the quotient obtained by dividing the dividend by the divisor.
leetcode Medium math bit-manipulation May 28, 2026
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LeetCode 30 - Substring with Concatenation of All Words

The problem asks us to find all starting indices in a string s where a substring exists that is a concatenation of all the words in the array words exactly once, without any intervening characters. The words can appear in any order, and all words in words have the same length.
leetcode Hard hash-table string sliding-window May 28, 2026
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LeetCode 31 - Next Permutation

The problem asks us to modify an integer array so that it becomes the next lexicographically greater permutation of its current arrangement. A permutation is simply an ordering of the elements. Lexicographical order works the same way dictionary order works for words.
leetcode Medium array two-pointers May 28, 2026
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LeetCode 32 - Longest Valid Parentheses

The problem asks us to find the length of the longest contiguous substring that forms a valid parentheses sequence. A valid parentheses sequence is one where every opening parenthesis '(' has a corresponding closing parenthesis ')', and the pairs are correctly nested.
leetcode Hard string dynamic-programming stack May 28, 2026
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LeetCode 33 - Search in Rotated Sorted Array

The problem gives us an array that was originally sorted in ascending order, but may have been rotated at some pivot point. A rotation means that some prefix of the sorted array was moved to the end while preserving the relative order of elements.
leetcode Medium array binary-search May 28, 2026
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LeetCode 34 - Find First and Last Position of Element in Sorted Array

The problem gives us a sorted integer array nums and a target value target. Our goal is to find the first position where the target appears and the last position where the target appears.
leetcode Medium array binary-search May 28, 2026
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LeetCode 35 - Search Insert Position

The problem gives us a sorted array of distinct integers and a target value. Our task is to determine where the target belongs in the array. If the target already exists, we return its index.
leetcode Easy array binary-search May 28, 2026
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LeetCode 36 - Valid Sudoku

This problem asks us to validate whether a partially filled Sudoku board follows the core Sudoku rules. The board is always a fixed 9 x 9 grid, and each cell contains either a digit from '1' to '9' or the character '.', which represents an empty cell.
leetcode Medium array hash-table matrix May 28, 2026
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LeetCode 37 - Sudoku Solver

The problem asks us to build a complete Sudoku solver. We are given a partially filled 9 x 9 grid where each cell contains either a digit from '1' to '9' or the character '.', which represents an empty space.
leetcode Hard array hash-table backtracking matrix May 28, 2026
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LeetCode 38 - Count and Say

The problem asks us to generate the nth term of the "count-and-say" sequence. This sequence is built recursively, where each term is created by describing the previous term.
leetcode Medium string May 28, 2026
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LeetCode 39 - Combination Sum

This problem asks us to generate every possible unique combination of numbers from the given candidates array such that the sum of the chosen numbers equals target. There are several important details in the problem statement: - Every number in candidates is distinct.
leetcode Medium array backtracking May 28, 2026
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LeetCode 40 - Combination Sum II

This problem asks us to find all unique combinations of numbers from the candidates array such that the sum of the chosen numbers equals target. Unlike the original Combination Sum problem, each number can only be used once.
leetcode Medium array backtracking May 28, 2026
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LeetCode 41 - First Missing Positive

The problem asks us to find the smallest positive integer that does not appear in an unsorted integer array. The key detail is that we only care about positive integers starting from 1.
leetcode Hard array hash-table May 28, 2026
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LeetCode 42 - Trapping Rain Water

The problem gives an array called height, where each element represents the height of a vertical bar in an elevation map. Every bar has width 1. After rainfall, water may become trapped between taller bars. The task is to compute the total amount of water that can be trapped.
leetcode Hard array two-pointers dynamic-programming stack monotonic-stack May 28, 2026
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LeetCode 43 - Multiply Strings

The problem asks us to multiply two non-negative integers where each integer is provided as a string instead of a numeric type. The result must also be returned as a string.
leetcode Medium math string simulation May 28, 2026
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LeetCode 44 - Wildcard Matching

This problem asks us to determine whether an entire input string s matches a wildcard pattern p. The pattern supports two special wildcard characters: - '?' matches exactly one character. - '' matches any sequence of characters, including an empty sequence.
leetcode Hard string dynamic-programming greedy recursion May 28, 2026
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LeetCode 45 - Jump Game II

The problem gives an array nums where each value represents the maximum distance you can jump forward from that position. You start at index 0, and your goal is to reach the final index using the minimum number of jumps possible.
leetcode Medium array dynamic-programming greedy May 28, 2026
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LeetCode 46 - Permutations

The problem gives us an array of distinct integers called nums, and asks us to generate every possible permutation of those numbers. A permutation is an arrangement of elements in a specific order.
leetcode Medium array backtracking May 28, 2026
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LeetCode 47 - Permutations II

The problem asks us to generate every possible permutation of the given array nums, while ensuring that duplicate permutations are not included in the final result. A permutation is an arrangement of all elements in a particular order.
leetcode Medium array backtracking sorting May 28, 2026
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LeetCode 48 - Rotate Image

The problem gives us an n x n square matrix that represents an image. Each element in the matrix corresponds to a pixel or value in the image. Our task is to rotate the entire image by 90 degrees clockwise. The important requirement is that the rotation must happen in-place.
leetcode Medium array math matrix May 28, 2026
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LeetCode 49 - Group Anagrams

The problem asks us to group together all strings that are anagrams of each other. Two strings are considered anagrams if they contain exactly the same characters with the same frequencies, but possibly in a different order.
leetcode Medium array hash-table string sorting May 28, 2026
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LeetCode 50 - Pow(x, n)

The problem asks us to implement exponentiation manually, specifically computing: where x is a floating point number and n is an integer exponent. The input consists of two values: - x, the base - n, the exponent The output should be the result of raising x to the power n.
leetcode Medium math recursion May 28, 2026
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LeetCode 51 - N-Queens

The N-Queens problem asks us to place n queens on an n x n chessboard such that no two queens can attack each other. In chess, a queen can attack horizontally, vertically, and diagonally.
leetcode Hard array backtracking May 28, 2026
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LeetCode 52 - N-Queens II

The n-queens puzzle asks us to place n queens on an n x n chessboard so that no two queens can attack each other. In chess, a queen can move horizontally, vertically, and diagonally.
leetcode Hard backtracking May 28, 2026
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LeetCode 53 - Maximum Subarray

The problem gives an integer array nums and asks us to find a contiguous subarray that has the largest possible sum. A subarray means the elements must appear next to each other in the original array. We are not allowed to rearrange elements or skip positions arbitrarily.
leetcode Medium array divide-and-conquer dynamic-programming May 28, 2026
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LeetCode 54 - Spiral Matrix

The problem gives an m x n matrix and asks us to return all elements in spiral order. Spiral order means we start from the top-left corner and move in a clockwise spiral pattern: 1. Traverse the top row from left to right 2. Traverse the right column from top to bottom 3.
leetcode Medium array matrix simulation May 28, 2026
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LeetCode 55 - Jump Game

The problem gives us an integer array nums, where each value represents the maximum distance we are allowed to jump forward from that position. You always start at index 0.
leetcode Medium array dynamic-programming greedy May 28, 2026
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LeetCode 56 - Merge Intervals

The problem gives an array of intervals, where each interval is represented as [start, end]. Each interval describes a continuous range of values from start to end, inclusive. The goal is to combine all intervals that overlap into a single larger interval.
leetcode Medium array sorting May 28, 2026
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LeetCode 57 - Insert Interval

The problem gives us a list of intervals that are already sorted by starting value and guaranteed to be non-overlapping. Each interval represents a continuous range, written as [start, end].
leetcode Medium array May 28, 2026
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LeetCode 58 - Length of Last Word

The problem gives us a string s that contains English letters and spaces. Inside this string, words are separated by spaces, and there may also be extra spaces at the beginning or end of the string. Our task is to return the length of the last word in the string.
leetcode Easy string May 28, 2026
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LeetCode 59 - Spiral Matrix II

The problem asks us to generate an n x n matrix and fill it with numbers from 1 to n^2 in spiral order. A spiral order traversal means we begin at the top-left corner and move in a clockwise pattern: 1. Move left to right across the top row 2.
leetcode Medium array matrix simulation May 28, 2026
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LeetCode 60 - Permutation Sequence

The problem gives us the numbers from 1 to n, and asks us to find the kth permutation when all possible permutations are ordered lexicographically.
leetcode Hard math recursion May 28, 2026
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LeetCode 61 - Rotate List

This problem asks us to rotate a singly linked list to the right by k positions. A right rotation means that each node shifts one position toward the end of the list, and the last node wraps around to become the new head.
leetcode Medium linked-list two-pointers May 28, 2026
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LeetCode 62 - Unique Paths

The problem describes a robot moving on a rectangular m x n grid. The robot starts at the top left corner of the grid and wants to reach the bottom right corner. At every step, the robot is only allowed to move either one cell to the right or one cell downward.
leetcode Medium math dynamic-programming combinatorics May 28, 2026
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LeetCode 63 - Unique Paths II

This problem asks us to count how many different valid paths exist for a robot moving through a grid while avoiding obstacles. The robot starts in the top-left corner of the grid at position (0, 0) and wants to reach the bottom-right corner at position (m - 1, n - 1).
leetcode Medium array dynamic-programming matrix May 28, 2026
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LeetCode 64 - Minimum Path Sum

The problem gives us a two dimensional grid of size m x n, where each cell contains a non-negative integer. We begin at the top-left corner of the grid, specifically at position (0, 0), and want to reach the bottom-right corner at position (m - 1, n - 1).
leetcode Medium array dynamic-programming matrix May 28, 2026
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LeetCode 65 - Valid Number

This is a comprehensive, multi section technical guide that will be quite long if completed properly with all requested sections, detailed explanations, two implementations, walkthroughs, worked examples, complexity analysis, exhaustive test cases, and edge case discussions.
leetcode Hard string May 28, 2026
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LeetCode 66 - Plus One

The problem gives us a non-empty array of digits that together represent a large integer. Each element in the array is a single decimal digit, and the digits are stored in left-to-right order from the most significant digit to the least significant digit.
leetcode Easy array math May 28, 2026
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LeetCode 67 - Add Binary

You are given two strings, a and b, where each string represents a binary number. A binary number contains only the characters '0' and '1'. The task is to add these two binary numbers together and return the result as another binary string.
leetcode Easy math string bit-manipulation simulation May 28, 2026
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LeetCode 68 - Text Justification

This problem asks us to simulate how text is formatted in a text editor when using full justification. We are given an array of words and a target line width called maxWidth.
leetcode Hard array string simulation May 28, 2026
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LeetCode 69 - Sqrt(x)

The problem asks us to compute the integer square root of a non negative integer x. More specifically, we need to return the largest integer k such that: This means we are not looking for the exact decimal square root. Instead, we must round down to the nearest integer.
leetcode Easy math binary-search May 28, 2026
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LeetCode 70 - Climbing Stairs

The problem describes a staircase with n total steps. You start at the bottom and want to reach the top. At every move, you are allowed to climb either 1 step or 2 steps. The goal is to determine how many distinct sequences of moves can take you from the bottom to the top.
leetcode Easy math dynamic-programming memoization May 28, 2026
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LeetCode 71 - Simplify Path

The problem gives us an absolute Unix-style file path and asks us to convert it into its canonical, simplified form. An absolute path always starts from the root directory, represented by /.
leetcode Medium string stack May 28, 2026
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LeetCode 72 - Edit Distance

The problem asks us to compute the minimum number of edit operations needed to transform one string into another. The allowed operations are insertion, deletion, and replacement of a single character. Each operation has a cost of exactly one.
leetcode Medium string dynamic-programming May 28, 2026
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LeetCode 73 - Set Matrix Zeroes

The problem gives us a two dimensional matrix of integers with m rows and n columns. We must modify the matrix in place so that whenever a cell contains 0, every element in that cell's entire row and entire column also becomes 0.
leetcode Medium array hash-table matrix May 28, 2026
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LeetCode 74 - Search a 2D Matrix

This problem gives us a two dimensional matrix with two very important ordering properties: 1. Each row is sorted from left to right in non-decreasing order. 2. The first element of every row is greater than the last element of the previous row.
leetcode Medium array binary-search matrix May 28, 2026
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LeetCode 75 - Sort Colors

This problem asks us to sort an array that contains only three distinct values: 0, 1, and 2. These values represent colors: - 0 represents red - 1 represents white - 2 represents blue The goal is to rearrange the array so that all 0s come first, followed by all 1s, followed by…
leetcode Medium array two-pointers sorting May 28, 2026
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LeetCode 76 - Minimum Window Substring

The problem asks us to find the smallest contiguous substring inside string s that contains every character from string t, including duplicate occurrences. A substring must consist of consecutive characters. We are not allowed to reorder characters or skip positions.
leetcode Hard hash-table string sliding-window May 28, 2026
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LeetCode 77 - Combinations

This problem asks us to generate all possible combinations of size k from the numbers in the inclusive range [1, n].
leetcode Medium backtracking May 28, 2026
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LeetCode 78 - Subsets

This problem asks us to generate every possible subset of a given array of unique integers. A subset is any selection of elements from the array, including the empty subset and the subset containing every element.
leetcode Medium array backtracking bit-manipulation May 28, 2026
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LeetCode 79 - Word Search

In this problem, we are given a two dimensional grid of characters called board and a target string called word. The task is to determine whether the word can be formed by traversing the grid under a strict set of movement rules.
leetcode Medium array string backtracking depth-first-search matrix May 28, 2026
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LeetCode 80 - Remove Duplicates from Sorted Array II

This problem asks us to modify a sorted integer array in-place so that every unique value appears at most twice. Since the array is already sorted in non-decreasing order, duplicate values will always appear next to each other, which is a very important property that makes an…
leetcode Medium array two-pointers May 28, 2026
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LeetCode 81 - Search in Rotated Sorted Array II

This problem asks us to determine whether a target value exists inside a rotated sorted array that may contain duplicate values. The original array was sorted in non-decreasing order, meaning values are arranged from smallest to largest, and duplicates are allowed.
leetcode Medium array binary-search May 27, 2026
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LeetCode 82 - Remove Duplicates from Sorted List II

This problem asks us to process a sorted singly linked list and remove every value that appears more than once. The important distinction is that we are not keeping one copy of duplicated values. Instead, every node containing a duplicated value must be removed entirely.
leetcode Medium linked-list two-pointers May 28, 2026
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LeetCode 83 - Remove Duplicates from Sorted List

This problem asks us to remove duplicate values from a sorted singly linked list so that every distinct value appears exactly once.
leetcode Easy linked-list May 28, 2026
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LeetCode 84 - Largest Rectangle in Histogram

This problem asks us to find the largest rectangular area that can be formed inside a histogram. A histogram is represented as an array of integers called heights, where each integer describes the height of a bar, and every bar has a width of exactly 1.
leetcode Hard array stack monotonic-stack May 28, 2026
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LeetCode 85 - Maximal Rectangle

The problem asks us to find the area of the largest rectangle containing only 1s in a binary matrix. The matrix consists of characters '0' and '1', where '1' represents a valid cell that may belong to a rectangle and '0' represents a blocked cell that cannot be included.
leetcode Hard array dynamic-programming stack matrix monotonic-stack May 28, 2026
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LeetCode 86 - Partition List

It looks like you want the full detailed solution guide, but the specific LeetCode problem number and statement are missing from your latest request template. You included the formatting instructions and structure, but not which problem to solve beyond prior context.
leetcode Medium linked-list two-pointers May 28, 2026
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LeetCode 87 - Scramble String

This problem asks us to determine whether one string can be transformed into another using a specific recursive scrambling process. We are given two strings, s1 and s2, and they are guaranteed to have the same length. The scrambling process works recursively.
leetcode Hard string dynamic-programming May 28, 2026
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LeetCode 88 - Merge Sorted Array

This problem gives us two arrays, nums1 and nums2, where both arrays are already sorted in non-decreasing order. We are also given two integers, m and n, which tell us how many valid elements exist in each array. The important detail is that nums1 has extra space at the end.
leetcode Easy array two-pointers sorting May 28, 2026
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LeetCode 89 - Gray Code

The problem asks us to generate a valid Gray code sequence for a given number of bits, n. A Gray code sequence is a special ordering of numbers where every adjacent pair differs by exactly one bit in binary form.
leetcode Medium math backtracking bit-manipulation May 28, 2026
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LeetCode 90 - Subsets II

The problem asks us to generate every possible subset of a given integer array. A subset is any selection of elements from the array, including the empty subset and the subset containing all elements.
leetcode Medium array backtracking bit-manipulation May 28, 2026
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LeetCode 91 - Decode Ways

This problem asks us to determine how many different valid ways a numeric string can be decoded into letters using the mapping: - "1" → 'A' - "2" → 'B' - ... - "26" → 'Z' The input is a string s containing only digits.
leetcode Medium string dynamic-programming May 28, 2026
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LeetCode 92 - Reverse Linked List II

This problem asks us to reverse only a specific portion of a singly linked list, rather than reversing the entire list. We are given the head of a linked list and two integer positions, left and right, where left <= right.
leetcode Medium linked-list May 28, 2026
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LeetCode 93 - Restore IP Addresses

The problem asks us to take a string containing only digits and determine every possible way to insert exactly three dots so that the resulting string becomes a valid IPv4 address. A valid IP address has four numeric segments separated by dots.
leetcode Medium string backtracking May 28, 2026
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LeetCode 94 - Binary Tree Inorder Traversal

This problem asks us to perform an inorder traversal on a binary tree and return the values of the visited nodes in the correct order. A binary tree is a hierarchical data structure where each node can have at most two children, a left child and a right child.
leetcode Easy stack tree depth-first-search binary-tree May 28, 2026
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LeetCode 95 - Unique Binary Search Trees II

The problem asks us to generate all structurally unique binary search trees (BSTs) that contain exactly n nodes labeled from 1 to n.
leetcode Medium dynamic-programming backtracking tree binary-search-tree binary-tree May 28, 2026
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LeetCode 96 - Unique Binary Search Trees

This problem asks us to determine how many structurally different Binary Search Trees, or BSTs, can be formed using the integers from 1 to n. A Binary Search Tree has an important property: - Every value in the left subtree is smaller than the root.
leetcode Medium math dynamic-programming tree binary-search-tree binary-tree May 28, 2026
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LeetCode 97 - Interleaving String

This problem asks us to determine whether a string s3 can be formed by interleaving two other strings, s1 and s2. An interleaving means we combine characters from s1 and s2 while preserving the relative order of characters within each original string.
leetcode Medium string dynamic-programming May 28, 2026
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LeetCode 98 - Validate Binary Search Tree

This problem asks us to determine whether a given binary tree satisfies the rules of a Binary Search Tree, commonly abbreviated as BST. A binary tree consists of nodes where each node contains a value and pointers to a left child and a right child.
leetcode Medium tree depth-first-search binary-search-tree binary-tree May 28, 2026
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LeetCode 99 - Recover Binary Search Tree

The problem gives us the root of a binary search tree, but exactly two nodes in the tree have had their values swapped accidentally. Our task is to restore the tree so that it once again satisfies the binary search tree property, without modifying the tree structure itself.
leetcode Medium tree depth-first-search binary-search-tree binary-tree May 28, 2026