| 4 | Median of Two Sorted Arrays | ArrayBinary SearchDivide and Conquer | Hard |
| 10 | Regular Expression Matching | StringDynamic ProgrammingRecursion | Hard |
| 23 | Merge k Sorted Lists | Linked ListDivide and ConquerHeap (Priority Queue)+1 | Hard |
| 25 | Reverse Nodes in k-Group | Linked ListRecursion | Hard |
| 30 | Substring with Concatenation of All Words | Hash TableStringSliding Window | Hard |
| 32 | Longest Valid Parentheses | StringDynamic ProgrammingStack | Hard |
| 37 | Sudoku Solver | ArrayHash TableBacktracking+1 | Hard |
| 41 | First Missing Positive | ArrayHash Table | Hard |
| 42 | Trapping Rain Water | ArrayTwo PointersDynamic Programming+2 | Hard |
| 44 | Wildcard Matching | StringDynamic ProgrammingGreedy+1 | Hard |
| 51 | N-Queens | ArrayBacktracking | Hard |
| 52 | N-Queens II | Backtracking | Hard |
| 60 | Permutation Sequence | MathRecursion | Hard |
| 65 | Valid Number | String | Hard |
| 68 | Text Justification | ArrayStringSimulation | Hard |
| 76 | Minimum Window Substring | Hash TableStringSliding Window | Hard |
| 84 | Largest Rectangle in Histogram | ArrayStackMonotonic Stack | Hard |
| 85 | Maximal Rectangle | ArrayDynamic ProgrammingStack+2 | Hard |
| 87 | Scramble String | StringDynamic Programming | Hard |
| 115 | Distinct Subsequences | StringDynamic Programming | Hard |
| 123 | Best Time to Buy and Sell Stock III | ArrayDynamic Programming | Hard |
| 124 | Binary Tree Maximum Path Sum | Dynamic ProgrammingTreeDepth-First Search+1 | Hard |
| 126 | Word Ladder II | Hash TableStringBacktracking+1 | Hard |
| 127 | Word Ladder | Hash TableStringBreadth-First Search | Hard |
| 132 | Palindrome Partitioning II | StringDynamic Programming | Hard |
| 135 | Candy | ArrayGreedy | Hard |
| 140 | Word Break II | ArrayHash TableString+4 | Hard |
| 149 | Max Points on a Line | ArrayHash TableMath+1 | Hard |
| 154 | Find Minimum in Rotated Sorted Array II | ArrayBinary Search | Hard |
| 174 | Dungeon Game | ArrayDynamic ProgrammingMatrix | Hard |
| 185 | Department Top Three Salaries | Database | Hard |
| 188 | Best Time to Buy and Sell Stock IV | ArrayDynamic Programming | Hard |
| 212 | Word Search II | ArrayStringBacktracking+2 | Hard |
| 214 | Shortest Palindrome | StringRolling HashString Matching+1 | Hard |
| 218 | The Skyline Problem | ArrayDivide and ConquerBinary Indexed Tree+4 | Hard |
| 220 | Contains Duplicate III | ArraySliding WindowSorting+2 | Hard |
| 224 | Basic Calculator | MathStringStack+1 | Hard |
| 233 | Number of Digit One | MathDynamic ProgrammingRecursion | Hard |
| 239 | Sliding Window Maximum | ArrayQueueSliding Window+2 | Hard |
| 262 | Trips and Users | Database | Hard |
| 273 | Integer to English Words | MathStringRecursion | Hard |
| 282 | Expression Add Operators | MathStringBacktracking | Hard |
| 295 | Find Median from Data Stream | Two PointersDesignSorting+2 | Hard |
| 297 | Serialize and Deserialize Binary Tree | StringTreeDepth-First Search+3 | Hard |
| 301 | Remove Invalid Parentheses | StringBacktrackingBreadth-First Search | Hard |
| 312 | Burst Balloons | ArrayDynamic Programming | Hard |
| 315 | Count of Smaller Numbers After Self | ArrayBinary SearchDivide and Conquer+4 | Hard |
| 321 | Create Maximum Number | ArrayTwo PointersStack+2 | Hard |
| 327 | Count of Range Sum | ArrayBinary SearchDivide and Conquer+4 | Hard |
| 329 | Longest Increasing Path in a Matrix | ArrayDynamic ProgrammingDepth-First Search+5 | Hard |