LeetCode 00xx
LeetCode practice notes for problems 1 through 99, including A clear explanation of the Two Sum problem using brute force first, then an optimized hash map solution.
LeetCode 00xx
| # | Title | Difficulty | Description |
|---|---|---|---|
| 1 | LeetCode 1: Two Sum | Easy | A clear explanation of the Two Sum problem using brute force first, then an optimized hash map solution. |
| 2 | LeetCode 2: Add Two Numbers | Medium | A detailed explanation of the Add Two Numbers linked list problem, including digit-by-digit addition, carry handling, and linked list construction. |
| 3 | LeetCode 3: Longest Substring Without Repeating Characters | Medium | A clear explanation of the longest substring problem using sliding window and a hash set. |
| 4 | LeetCode 4: Median of Two Sorted Arrays | Hard | A detailed explanation of finding the median of two sorted arrays using binary search over partitions. |
| 5 | LeetCode 5: Longest Palindromic Substring | Medium | A detailed explanation of finding the longest palindromic substring using expand-around-center. |
| 6 | LeetCode 6: Zigzag Conversion | Medium | A detailed explanation of converting a string into a zigzag pattern using row simulation. |
| 7 | LeetCode 7: Reverse Integer | Medium | A detailed explanation of reversing a signed 32-bit integer while handling overflow correctly. |
| 8 | LeetCode 8: String to Integer (atoi) | Medium | A detailed explanation of parsing a string into a 32-bit signed integer with whitespace, sign, digit reading, and clamping rules. |
| 9 | LeetCode 9: Palindrome Number | Easy | A detailed explanation of checking whether an integer is a palindrome using digit operations without converting it to a string. |
| 10 | LeetCode 10: Regular Expression Matching | Hard | A detailed explanation of matching a full string against a simplified regular expression with dot and star using dynamic programming. |
| 11 | LeetCode 11: Container With Most Water | Medium | A detailed explanation of finding the maximum water container area using two pointers. |
| 12 | LeetCode 12: Integer to Roman | Medium | A detailed explanation of converting an integer into a Roman numeral using a fixed value-symbol table and greedy subtraction. |
| 13 | LeetCode 13: Roman to Integer | Easy | A detailed explanation of converting a Roman numeral string into an integer using symbol values and the subtraction rule. |
| 14 | LeetCode 14: Longest Common Prefix | Easy | A detailed explanation of finding the longest common prefix among an array of strings by comparing characters column by column. |
| 15 | LeetCode 15: 3Sum | Medium | A detailed explanation of finding all unique triplets that sum to zero using sorting and two pointers. |
| 16 | LeetCode 16: 3Sum Closest | Medium | A detailed explanation of finding the sum of three integers closest to a target using sorting and two pointers. |
| 17 | LeetCode 17: Letter Combinations of a Phone Number | Medium | A detailed explanation of generating all possible phone keypad letter combinations using backtracking. |
| 18 | LeetCode 18: 4Sum | Medium | A detailed explanation of finding all unique quadruplets that sum to a target using sorting and two pointers. |
| 19 | LeetCode 19: Remove Nth Node From End of List | Medium | A detailed explanation of removing the nth node from the end of a singly linked list using two pointers and a dummy node. |
| 20 | LeetCode 20: Valid Parentheses | Easy | A detailed explanation of checking whether a bracket string is valid using a stack. |
| 21 | LeetCode 21: Merge Two Sorted Lists | Easy | A detailed explanation of merging two sorted linked lists using a dummy node and pointer splicing. |
| 22 | LeetCode 22: Generate Parentheses | Medium | A detailed explanation of generating all well-formed parentheses strings using backtracking. |
| 23 | LeetCode 23: Merge k Sorted Lists | Hard | A detailed explanation of merging k sorted linked lists using a min heap. |
| 24 | LeetCode 24: Swap Nodes in Pairs | Medium | A detailed explanation of swapping every two adjacent nodes in a linked list using pointer manipulation. |
| 25 | LeetCode 25: Reverse Nodes in k-Group | Hard | A detailed explanation of reversing linked-list nodes in groups of k using pointer manipulation and constant extra space. |
| 26 | LeetCode 26: Remove Duplicates from Sorted Array | Easy | A clear explanation of removing duplicates from a sorted array in place using two pointers. |
| 27 | LeetCode 27: Remove Element | Easy | A clear explanation of removing all occurrences of a value from an array in place using a write pointer. |
| 28 | LeetCode 28: Find the Index of the First Occurrence in a String | Easy | A clear explanation of finding the first occurrence of one string inside another using direct string matching. |
| 29 | LeetCode 29: Divide Two Integers | Medium | A clear explanation of integer division without using multiplication, division, or modulo, using repeated doubling with bit shifts. |
| 30 | LeetCode 30: Substring with Concatenation of All Words | Hard | A clear explanation of finding all starting indices where a substring is formed by concatenating every word exactly once. |
| 31 | LeetCode 31: Next Permutation | Medium | A clear explanation of finding the next lexicographically greater permutation in place using a right-to-left scan. |
| 32 | LeetCode 32: Longest Valid Parentheses | Hard | A clear explanation of finding the longest well-formed parentheses substring using a stack of indices. |
| 33 | LeetCode 33: Search in Rotated Sorted Array | Medium | A clear explanation of searching a rotated sorted array in logarithmic time using modified binary search. |
| 34 | LeetCode 34: Find First and Last Position of Element in Sorted Array | Medium | A clear explanation of finding the first and last index of a target in a sorted array using two binary searches. |
| 35 | LeetCode 35: Search Insert Position | Easy | A clear explanation of finding the index of a target, or where it should be inserted, using binary search. |
| 36 | LeetCode 36: Valid Sudoku | Medium | A clear explanation of checking whether a partially filled Sudoku board is valid using hash sets. |
| 37 | LeetCode 37: Sudoku Solver | Hard | A clear explanation of solving a Sudoku board using backtracking and constraint checking. |
| 38 | LeetCode 38: Count and Say | Medium | A clear explanation of generating the count-and-say sequence using run-length encoding. |
| 39 | LeetCode 39: Combination Sum | Medium | A clear explanation of finding all unique combinations that sum to a target using backtracking. |
| 40 | LeetCode 40: Combination Sum II | Medium | A clear explanation of finding unique combinations that sum to a target when each array element may be used at most once. |
| 41 | LeetCode 41: First Missing Positive | Hard | A clear explanation of the First Missing Positive problem using in-place index placement to achieve O(n) time and O(1) extra space. |
| 42 | LeetCode 42: Trapping Rain Water | Hard | A clear explanation of the Trapping Rain Water problem using left and right boundaries, then an optimized two-pointer solution. |
| 43 | LeetCode 43: Multiply Strings | Medium | A clear explanation of Multiply Strings using grade-school multiplication with digit arrays. |
| 44 | LeetCode 44: Wildcard Matching | Hard | A clear explanation of Wildcard Matching using dynamic programming over string and pattern prefixes. |
| 45 | LeetCode 45: Jump Game II | Medium | A clear explanation of Jump Game II using a greedy range expansion approach to find the minimum number of jumps. |
| 46 | LeetCode 46: Permutations | Medium | A clear explanation of Permutations using depth-first search and backtracking. |
| 47 | LeetCode 47: Permutations II | Medium | A clear explanation of Permutations II using sorting, depth-first search, and duplicate-skipping backtracking. |
| 48 | LeetCode 48: Rotate Image | Medium | A clear explanation of Rotate Image using in-place matrix transpose and row reversal. |
| 49 | LeetCode 49: Group Anagrams | Medium | A clear explanation of Group Anagrams using a hash map keyed by each word's sorted character signature. |
| 50 | LeetCode 50: Pow(x, n) | Medium | A clear explanation of Pow(x, n) using binary exponentiation to compute powers in logarithmic time. |
| 51 | LeetCode 51: N-Queens | Hard | A clear guide to solving N-Queens with backtracking, row-by-row placement, and constant-time conflict checks. |
| 52 | LeetCode 52: N-Queens II | Hard | A clear guide to solving N-Queens II by counting valid queen placements with backtracking. |
| 53 | LeetCode 53: Maximum Subarray | Medium | A clear guide to solving Maximum Subarray with brute force first, then Kadane's dynamic programming algorithm. |
| 54 | LeetCode 54: Spiral Matrix | Medium | A clear guide to reading a matrix in spiral order using shrinking boundaries. |
| 55 | LeetCode 55: Jump Game | Medium | A clear guide to solving Jump Game with greedy reachability. |
| 56 | LeetCode 56: Merge Intervals | Medium | A clear guide to solving Merge Intervals by sorting intervals and merging them in one pass. |
| 57 | LeetCode 57: Insert Interval | Medium | A clear guide to solving Insert Interval with one linear scan over sorted, non-overlapping intervals. |
| 58 | LeetCode 58: Length of Last Word | Easy | A clear guide to solving Length of Last Word by scanning the string from right to left. |
| 59 | LeetCode 59: Spiral Matrix II | Medium | A clear guide to generating an n x n matrix filled from 1 to n squared in spiral order. |
| 60 | LeetCode 60: Permutation Sequence | Hard | A clear guide to finding the kth permutation sequence using factorial blocks instead of generating all permutations. |
| 61 | LeetCode 61: Rotate List | Medium | A clear guide to rotating a linked list to the right by k places using a circular list. |
| 62 | LeetCode 62: Unique Paths | Medium | A clear guide to counting unique paths in a grid using dynamic programming. |
| 63 | LeetCode 63: Unique Paths II | Medium | A clear guide to counting unique paths in a grid with obstacles using dynamic programming. |
| 64 | LeetCode 64: Minimum Path Sum | Medium | A clear guide to finding the minimum path sum in a grid using dynamic programming. |
| 65 | LeetCode 65: Valid Number | Hard | A clear guide to validating whether a string is a valid number using grammar rules and one left-to-right scan. |
| 66 | LeetCode 66: Plus One | Easy | A clear guide to adding one to a large integer represented as an array of digits. |
| 67 | LeetCode 67: Add Binary | Easy | A clear guide to adding two binary strings using two pointers and a carry. |
| 68 | LeetCode 68: Text Justification | Hard | A clear guide to formatting text with greedy line packing and even space distribution. |
| 69 | LeetCode 69: Sqrt(x) | Easy | A clear guide to computing the integer square root using binary search without built-in exponent functions. |
| 70 | LeetCode 70: Climbing Stairs | Easy | A clear guide to counting distinct ways to climb stairs using dynamic programming. |
| 71 | LeetCode 71: Simplify Path | Medium | A clear guide to simplifying Unix-style file paths using a stack. |
| 72 | LeetCode 72: Edit Distance | Hard | A clear guide to computing the minimum number of insert, delete, and replace operations needed to convert one string into another. |
| 73 | LeetCode 73: Set Matrix Zeroes | Medium | A clear guide to setting matrix rows and columns to zero in place using the first row and first column as markers. |
| 74 | LeetCode 74: Search a 2D Matrix | Medium | A clear guide to searching a sorted 2D matrix using binary search over a virtual one-dimensional array. |
| 75 | LeetCode 75: Sort Colors | Medium | A clear guide to sorting an array of 0s, 1s, and 2s in place using the Dutch National Flag algorithm. |
| 76 | LeetCode 76: Minimum Window Substring | Hard | A detailed guide to solving Minimum Window Substring with a sliding window and frequency counters. |
| 77 | LeetCode 77: Combinations | Medium | A detailed guide to solving Combinations with backtracking and pruning. |
| 78 | LeetCode 78: Subsets | Medium | A detailed guide to solving Subsets with backtracking and the include-or-skip recursion idea. |
| 79 | LeetCode 79: Word Search | Medium | A detailed guide to solving Word Search with depth-first search and backtracking on a grid. |
| 80 | LeetCode 80: Remove Duplicates from Sorted Array II | Medium | A detailed guide to solving Remove Duplicates from Sorted Array II with an in-place two-pointer method. |
| 81 | LeetCode 81: Search in Rotated Sorted Array II | Medium | A detailed guide to solving Search in Rotated Sorted Array II with modified binary search and duplicate handling. |
| 82 | LeetCode 82: Remove Duplicates from Sorted List II | Medium | A detailed guide to solving Remove Duplicates from Sorted List II with a dummy node and pointer rewiring. |
| 83 | LeetCode 83: Remove Duplicates from Sorted List | Easy | A detailed guide to solving Remove Duplicates from Sorted List with one pointer and in-place linked list rewiring. |
| 84 | LeetCode 84: Largest Rectangle in Histogram | Hard | A detailed guide to solving Largest Rectangle in Histogram with a monotonic increasing stack. |
| 85 | LeetCode 85: Maximal Rectangle | Hard | A detailed guide to solving Maximal Rectangle by converting each matrix row into a histogram and applying a monotonic stack. |
| 86 | LeetCode 86: Partition List | Medium | A detailed guide to solving Partition List with two dummy lists while preserving relative order. |
| 87 | LeetCode 87: Scramble String | Hard | A detailed guide to solving Scramble String with recursive dynamic programming and memoization. |
| 88 | LeetCode 88: Merge Sorted Array | Easy | A detailed guide to solving Merge Sorted Array in-place by merging from the back with three pointers. |
| 89 | LeetCode 89: Gray Code | Medium | A detailed guide to solving Gray Code using the binary-to-Gray-code formula. |
| 90 | LeetCode 90: Subsets II | Medium | A detailed guide to solving Subsets II with sorting, backtracking, and duplicate skipping. |
| 91 | LeetCode 91: Decode Ways | Medium | A detailed guide to solving Decode Ways with dynamic programming and careful handling of zeroes. |
| 92 | LeetCode 92: Reverse Linked List II | Medium | A detailed guide to solving Reverse Linked List II with a dummy node and in-place sublist reversal. |
| 93 | LeetCode 93: Restore IP Addresses | Medium | A detailed guide to solving Restore IP Addresses with backtracking over four valid IP segments. |
| 94 | LeetCode 94: Binary Tree Inorder Traversal | Easy | A detailed guide to solving Binary Tree Inorder Traversal with recursion and an iterative stack. |
| 95 | LeetCode 95: Unique Binary Search Trees II | Medium | A detailed guide to solving Unique Binary Search Trees II with recursive tree generation over value ranges. |
| 96 | LeetCode 96: Unique Binary Search Trees | Medium | A detailed guide to solving Unique Binary Search Trees with dynamic programming and the Catalan recurrence. |
| 97 | LeetCode 97: Interleaving String | Medium | A detailed guide to solving Interleaving String with two-dimensional dynamic programming. |
| 98 | LeetCode 98: Validate Binary Search Tree | Medium | A detailed guide to solving Validate Binary Search Tree with recursive lower and upper bounds. |
| 99 | LeetCode 99: Recover Binary Search Tree | Medium | A detailed guide to solving Recover Binary Search Tree with inorder traversal and two misplaced nodes. |