| 4 | Median of Two Sorted Arrays | Hard |
| 10 | Regular Expression Matching | Hard |
| 23 | Merge k Sorted Lists | Hard |
| 25 | Reverse Nodes in k-Group | Hard |
| 30 | Substring with Concatenation of All Words | Hard |
| 32 | Longest Valid Parentheses | Hard |
| 37 | Sudoku Solver | Hard |
| 41 | First Missing Positive | Hard |
| 42 | Trapping Rain Water | Hard |
| 44 | Wildcard Matching | Hard |
| 45 | Jump Game II | Hard |
| 51 | N-Queens | Hard |
| 52 | N-Queens II | Hard |
| 57 | Insert Interval | Hard |
| 65 | Valid Number | Hard |
| 68 | Text Justification | Hard |
| 72 | Edit Distance | Hard |
| 76 | Minimum Window Substring | Hard |
| 84 | Largest Rectangle in Histogram | Hard |
| 85 | Maximal Rectangle | Hard |
| 87 | Scramble String | Hard |
| 97 | Interleaving String | Hard |
| 99 | Recover Binary Search Tree | Hard |
| 115 | Distinct Subsequences | Hard |
| 123 | Best Time to Buy and Sell Stock III | Hard |
| 124 | Binary Tree Maximum Path Sum | Hard |
| 126 | Word Ladder II | Hard |
| 128 | Longest Consecutive Sequence | Hard |
| 132 | Palindrome Partitioning II | Hard |
| 135 | Candy | Hard |