| 20 | Valid Parentheses | Easy |
| 42 | Trapping Rain Water | Hard |
| 71 | Simplify Path | Medium |
| 84 | Largest Rectangle in Histogram | Hard |
| 85 | Maximal Rectangle | Hard |
| 94 | Binary Tree Inorder Traversal | Medium |
| 103 | Binary Tree Zigzag Level Order Traversal | Medium |
| 144 | Binary Tree Preorder Traversal | Medium |
| 145 | Binary Tree Postorder Traversal | Hard |
| 150 | Evaluate Reverse Polish Notation | Medium |
| 155 | Min Stack | Easy |
| 173 | Binary Search Tree Iterator | Medium |
| 224 | Basic Calculator | Hard |
| 225 | Implement Stack using Queues | Easy |
| 227 | Basic Calculator II | Medium |
| 232 | Implement Queue using Stacks | Easy |
| 316 | Remove Duplicate Letters | Hard |
| 321 | Create Maximum Number | Hard |
| 331 | Verify Preorder Serialization of a Binary Tree | Medium |
| 341 | Flatten Nested List Iterator | Medium |
| 378 | Kth Smallest Element in a Sorted Matrix | Medium |
| 385 | Mini Parser | Medium |
| 388 | Longest Absolute File Path | Medium |
| 394 | Decode String | Medium |
| 402 | Remove K Digits | Medium |
| 445 | Add Two Numbers II | Medium |
| 456 | 132 Pattern | Medium |
| 496 | Next Greater Element I | Easy |
| 503 | Next Greater Element II | Medium |
| 581 | Shortest Unsorted Continuous Subarray | Medium |