| 1 | Balanced Binary Tree: Height Check with Early Exit | Easy | coding | |
| 2 | Best Time to Buy and Sell Stock: Maximum Profit From One Trade | Easy | coding | |
| 3 | Binary Search: Find a Target in a Sorted Array in O(log n) | Easy | coding | |
| 4 | Climbing Stairs: Count Ways with a Fibonacci-Style DP | Easy | coding | |
| 5 | Contains Duplicate: Detect Whether Any Value Appears Twice | Easy | coding | |
| 6 | Convert Sorted Array to BST: Pick the Middle, Recurse on Halves | Easy | coding | |
| 7 | Diameter of Binary Tree: Longest Path via Post-Order Heights | Easy | coding | |
| 8 | Implement Queue using Stacks: FIFO With Two Stacks in Amortised O(1) | Easy | coding | |
| 9 | Implement Stack using Queues: LIFO Behaviour From FIFO Parts | Easy | coding | |
| 10 | Invert Binary Tree: Recursive and Iterative Mirror Solutions | Easy | coding | |
| 11 | Kth Largest Element in a Stream: Size-k Min-Heap | Easy | coding | |
| 12 | Last Stone Weight: Simulating Smashes with a Max-Heap | Easy | coding | |
| 13 | Linked List Cycle: Floyd's Fast and Slow Pointers | Easy | coding | |
| 14 | Longest Common Prefix: Shared Start of a List of Strings | Easy | coding | |
| 15 | Majority Element: Find the Value That Fills More Than Half the Array | Easy | coding | |
| 16 | Maximum Average Subarray I: Best Average Over a Fixed-Length Window | Easy | coding | |
| 17 | Maximum Depth of Binary Tree: Recursive DFS, Iterative DFS and BFS | Easy | coding | |
| 18 | Merge Two Sorted Lists: Dummy Head Iteration and Recursion | Easy | coding | |
| 19 | Min Cost Climbing Stairs: Cheapest Path to the Top with 1D DP | Easy | coding | |
| 20 | Move Zeroes: Shift Zeros to the End While Keeping Order | Easy | coding | |
| 21 | Next Greater Element I: First Larger Value to the Right | Easy | coding | |
| 22 | Number of 1 Bits: Count Set Bits in an Integer | Easy | coding | |
| 23 | Number of Recent Calls: Count Requests in a Sliding Time Window | Easy | coding | |
| 24 | Reverse Bits: Mirror the 32 Bits of an Unsigned Integer | Easy | coding | |
| 25 | Reverse Linked List: Iterative and Recursive Solutions | Easy | coding | |
| 26 | Reverse String: Reverse a Character Array in Place | Easy | coding | |
| 27 | Same Tree: Comparing Two Binary Trees Recursively and Iteratively | Easy | coding | |
| 28 | Single Number: Find the Value That Appears Only Once Using XOR | Easy | coding | |
| 29 | Subtree of Another Tree: Brute-Force Matching and Linear Serialisation | Easy | coding | |
| 30 | Two Sum: Find Two Indices That Add Up to a Target | Easy | coding | |
| 31 | Valid Anagram: Check Whether Two Strings Use the Same Letters | Easy | coding | |
| 32 | Valid Palindrome II: Palindrome After Deleting at Most One Character | Easy | coding | |
| 33 | Valid Palindrome: Check a Phrase While Ignoring Case and Punctuation | Easy | coding | |
| 34 | Valid Parentheses: Check That Every Bracket Closes in the Right Order | Easy | coding | |
| 35 | 3Sum: Find All Unique Triplets That Sum to Zero | Medium | coding | |
| 36 | Add Two Numbers: Digit-by-Digit Addition on Linked Lists | Medium | coding | |
| 37 | Best Time to Buy and Sell Stock with Cooldown: State-Machine DP | Medium | coding | |
| 38 | Binary Tree Level Order Traversal: BFS with a Queue, Level by Level | Medium | coding | |
| 39 | Binary Tree Right Side View: Last Node of Each Level | Medium | coding | |
| 40 | Car Fleet: Count the Groups of Cars Arriving at a Destination | Medium | coding | |
| 41 | Cheapest Flights Within K Stops: Bounded Bellman-Ford for Limited Hops | Medium | coding | |
| 42 | Clone Graph: Deep-Copy a Connected Graph with a Hash Map and BFS or DFS | Medium | coding | |
| 43 | Coin Change II: Count Combinations That Make an Amount | Medium | coding | |
| 44 | Coin Change: Fewest Coins with Bottom-Up DP | Medium | coding | |
| 45 | Combination Sum II: Use Each Value Once and Avoid Duplicate Answers | Medium | coding | |
| 46 | Combination Sum: Reach a Target with Reusable Values via Backtracking | Medium | coding | |
| 47 | Construct Binary Tree from Preorder and Inorder Traversal | Medium | coding | |
| 48 | Container With Most Water: Maximise Area Between Two Lines | Medium | coding | |
| 49 | Copy List with Random Pointer: Hash Map and Interleaving Solutions | Medium | coding | |
| 50 | Count Good Nodes in Binary Tree: DFS Carrying the Path Maximum | Medium | coding | |
| 51 | Course Schedule II: Return a Valid Course Order with Topological Sort | Medium | coding | |
| 52 | Course Schedule: Detect a Cycle in a Prerequisite Graph with Kahn's Algorithm | Medium | coding | |
| 53 | Daily Temperatures: Days Until a Warmer Day With a Monotonic Stack | Medium | coding | |
| 54 | Decode Ways: Count Digit-to-Letter Decodings with Prefix DP | Medium | coding | |
| 55 | Delete Node in a BST: Leaf, One Child and Two Children Cases | Medium | coding | |
| 56 | Design Add and Search Words: Trie Search with Wildcards | Medium | coding, architecture | |
| 57 | Design Circular Queue: A Fixed-Capacity Ring Buffer | Medium | coding | |
| 58 | Design Twitter: News Feed with a K-Way Heap Merge | Medium | coding, architecture | |
| 59 | Edit Distance: Levenshtein Distance with a 2D DP Table | Medium | coding | |
| 60 | Encode and Decode Strings: Serialise a List of Strings Safely | Medium | coding | |
| 61 | Evaluate Reverse Polish Notation: Compute a Postfix Expression | Medium | coding | |
| 62 | Find First and Last Position of a Value: Lower and Upper Bound Binary Search | Medium | coding | |
| 63 | Find Minimum in Rotated Sorted Array: Binary Search Against the Right End | Medium | coding | |
| 64 | Find the Duplicate Number: Cycle Detection on an Array | Medium | coding | |
| 65 | Generate Parentheses: List Every Balanced String of n Pairs | Medium | coding | |
| 66 | Graph Valid Tree: Check Edge Count, Connectivity and Cycles with Union-Find | Medium | coding | |
| 67 | Group Anagrams: Bucket Words That Share the Same Letters | Medium | coding | |
| 68 | House Robber II: Non-Adjacent Maximum Sum When Houses Form a Circle | Medium | coding | |
| 69 | House Robber: Maximum Non-Adjacent Sum with Take-or-Skip DP | Medium | coding | |
| 70 | Implement Trie (Prefix Tree): Insert, Search and Prefix Lookup | Medium | coding, architecture | |
| 71 | Insert Interval: Add a Range to a Sorted List and Merge Overlaps | Medium | coding | |
| 72 | Insert into a BST: Walk Down to the Empty Spot | Medium | coding | |
| 73 | Interleaving String: Can Two Strings Merge into a Third? 2D DP | Medium | coding | |
| 74 | K Closest Points to Origin: Bounded Max-Heap and Quickselect | Medium | coding | |
| 75 | Koko Eating Bananas: Binary Search on the Answer | Medium | coding | |
| 76 | Kth Largest Element in an Array: Heap, Quickselect and Counting | Medium | coding | |
| 77 | Kth Smallest Element in a BST: Inorder Traversal with Early Stop | Medium | coding | |
| 78 | Letter Combinations of a Phone Number: Cartesian Product by Backtracking | Medium | coding | |
| 79 | Longest Common Subsequence: The Classic Two-String DP Table | Medium | coding | |
| 80 | Longest Consecutive Sequence: Longest Run of Consecutive Integers in O(n) | Medium | coding | |
| 81 | Longest Increasing Subsequence: O(n^2) DP and O(n log n) Patience Sorting | Medium | coding | |
| 82 | Longest Palindromic Substring: Expand Around Centres or Interval DP | Medium | coding | |
| 83 | Longest Repeating Character Replacement: Window With At Most k Changes | Medium | coding | |
| 84 | Longest Substring Without Repeating Characters: Sliding Window | Medium | coding | |
| 85 | Lowest Common Ancestor of a Binary Tree: One Post-Order Search | Medium | coding | |
| 86 | Lowest Common Ancestor of a BST: Follow the Split Point | Medium | coding | |
| 87 | LRU Cache: Hash Map Plus Doubly Linked List for O(1) Operations | Medium | coding, architecture | |
| 88 | Max Area of Island: Largest Connected Land Region with Flood Fill | Medium | coding | |
| 89 | Maximum Product Subarray: Track Both the Largest and Smallest Product | Medium | coding | |
| 90 | Maximum Subarray: Largest Sum of a Contiguous Slice with Kadane's Algorithm | Medium | coding | |
| 91 | Merge Intervals: Combine Every Overlapping Range | Medium | coding | |
| 92 | Min Cost to Connect All Points: Minimum Spanning Tree with Prim or Kruskal | Medium | coding | |
| 93 | Min Stack: A Stack That Returns Its Minimum in Constant Time | Medium | coding | |
| 94 | Network Delay Time: Single-Source Shortest Paths with Dijkstra's Algorithm | Medium | coding | |
| 95 | Non-overlapping Intervals: Fewest Removals to Eliminate Overlaps | Medium | coding | |
| 96 | Number of Connected Components in an Undirected Graph with Union-Find | Medium | coding | |
| 97 | Number of Islands: Count Connected Land Cells with BFS, DFS or Union-Find | Medium | coding | |
| 98 | Pacific Atlantic Water Flow: Reverse Multi-Source BFS from Both Oceans | Medium | coding | |
| 99 | Palindrome Partitioning: Split a String into Palindromes by Backtracking | Medium | coding | |
| 100 | Palindromic Substrings: Count Every Palindrome by Expanding Around Centres | Medium | coding | |
| 101 | Partition Equal Subset Sum: 0/1 Knapsack on Half the Total | Medium | coding | |
| 102 | Permutation in String: Does One String Contain a Rearrangement of Another? | Medium | coding | |
| 103 | Permutations: Every Ordering of Distinct Values with Backtracking | Medium | coding | |
| 104 | Product of Array Except Self: Prefix and Suffix Products Without Division | Medium | coding | |
| 105 | Redundant Connection: Find the Edge That Closes a Cycle with Union-Find | Medium | coding | |
| 106 | Remove Nth Node From End of List: One Pass with a Gap of n | Medium | coding | |
| 107 | Reorder List: Find the Middle, Reverse, Then Interleave | Medium | coding | |
| 108 | Rotate Image: Turn a Square Matrix 90 Degrees Clockwise in Place | Medium | coding | |
| 109 | Rotting Oranges: Minutes to Spread via Level-by-Level Multi-Source BFS | Medium | coding | |
| 110 | Search a 2D Matrix: Binary Search over a Flattened Sorted Grid | Medium | coding | |
| 111 | Search in Rotated Sorted Array: One-Pass Binary Search | Medium | coding | |
| 112 | Set Matrix Zeroes: Zero Out Rows and Columns in Place | Medium | coding | |
| 113 | Sort Colors: One-Pass Three-Way Partition of 0s, 1s and 2s | Medium | coding | |
| 114 | Spiral Matrix: Read a Grid in Clockwise Spiral Order | Medium | coding | |
| 115 | String to Integer (atoi): Parse a Signed 32-bit Integer by Hand | Medium | coding | |
| 116 | Subarray Sum Equals K: Count Contiguous Slices With a Given Sum | Medium | coding | |
| 117 | Subsets II: Power Set Without Duplicates When Values Repeat | Medium | coding | |
| 118 | Subsets: Generate the Power Set with Include/Exclude Backtracking | Medium | coding | |
| 119 | Surrounded Regions: Capture Enclosed Cells by Flood-Filling from the Border | Medium | coding | |
| 120 | Target Sum: Count Sign Assignments with Subset-Sum DP | Medium | coding | |
| 121 | Task Scheduler: Cooldowns with a Max-Heap or a Counting Formula | Medium | coding | |
| 122 | Time Based Key-Value Store: Versioned Lookups with Binary Search | Medium | coding, architecture | |
| 123 | Top K Frequent Elements: Return the k Most Common Values | Medium | coding | |
| 124 | Two Sum II: Find a Pair With a Target Sum in a Sorted Array | Medium | coding | |
| 125 | Unique Paths: Count Grid Routes with 2D DP or a Binomial Coefficient | Medium | coding | |
| 126 | Valid Sudoku: Check Rows, Columns and Boxes for Repeated Digits | Medium | coding | |
| 127 | Validate Binary Search Tree: Bounds Recursion and Inorder Check | Medium | coding | |
| 128 | Word Break: Can a String Be Split into Dictionary Words? Prefix DP | Medium | coding | |
| 129 | Word Search: Trace a Word Through a Letter Grid with DFS Backtracking | Medium | coding | |
| 130 | Alien Dictionary: Derive a Letter Order from Sorted Words with Topological Sort | Hard | coding | |
| 131 | Binary Tree Maximum Path Sum: Post-Order Gains with a Global Best | Hard | coding | |
| 132 | Burst Balloons: Interval DP by Choosing the Last Balloon to Burst | Hard | coding | |
| 133 | Find Median from Data Stream: Two Heaps Kept in Balance | Hard | coding | |
| 134 | Largest Rectangle in Histogram: Biggest Area Under the Bars | Hard | coding | |
| 135 | Median of Two Sorted Arrays: Partition Binary Search in O(log min(m, n)) | Hard | coding | |
| 136 | Merge k Sorted Lists: Min-Heap and Divide-and-Conquer Solutions | Hard | coding | |
| 137 | Minimum Window Substring: Shortest Slice Containing Every Required Character | Hard | coding | |
| 138 | N-Queens: Place Non-Attacking Queens Row by Row with Backtracking | Hard | coding | |
| 139 | Reconstruct Itinerary: Eulerian Path with Hierholzer's Algorithm | Hard | coding | |
| 140 | Regular Expression Matching: Dot and Star with a 2D DP Table | Hard | coding | |
| 141 | Reverse Nodes in k-Group: In-Place Group Reversal on a Linked List | Hard | coding | |
| 142 | Serialize and Deserialize Binary Tree: Preorder with Null Markers | Hard | coding | |
| 143 | Sliding Window Maximum: Max of Every Window With a Monotonic Deque | Hard | coding | |
| 144 | Swim in Rising Water: Minimax Path with a Modified Dijkstra | Hard | coding | |
| 145 | Trapping Rain Water: Total Water Held Between Elevation Bars | Hard | coding | |
| 146 | Word Ladder: Shortest Word Transformation with BFS and Wildcard Buckets | Hard | coding | |
| 147 | Word Search II: Find Many Words in a Grid with a Trie and Backtracking | Hard | coding | |