A collection of LeetCode Hard difficulty problems solved with step-by-step explanations, pattern categorization, and optimized source code.
| # | Problem | Pattern | Language | Time | Space |
|---|---|---|---|---|---|
| 4 | Median of Two Sorted Arrays | Binary Search | Python | O(n) | O(1) |
| 10 | Regular Expression Matching | Dynamic Programming | JavaScript | O(n) | O(n) |
| 23 | Merge k Sorted Lists | Two Pointers | Python | O(n) | O(1) |
| 25 | Reverse Nodes in k-Group | Two Pointers | Python | O(n) | O(1) |
| 30 | Substring with Concatenation of All Words | Sliding Window | Python | O(n) | O(1) |
| 32 | Longest Valid Parentheses | Stack & Queue | Python | O(n) | O(n) |
| 37 | Sudoku Solver | Array / General | Python | O(n) | O(1) |
| 41 | First Missing Positive | Array / General | Python | O(n) | O(1) |
| 42 | Trapping Rain Water | Array / General | Python | O(n) | O(1) |
| 44 | Wildcard Matching | Array / General | Python | O(n) | O(1) |
| 51 | N-Queens | Tree & Graph | Python | O(n) | O(n) |
| 52 | N-Queens II | Array / General | Python | O(n) | O(1) |
| 60 | Permutation Sequence | Backtracking | Python | O(n) | O(1) |
| 65 | Valid Number | Array / General | Python | O(n) | O(1) |
| 68 | Text Justification | Heap | Python | O(n) | O(n) |
| 76 | Minimum Window Substring | Sliding Window | PHP | O(n) | O(1) |
| 84 | Largest Rectangle in Histogram | Monotonic Stack | TypeScript | O(n) | O(n) |
| 85 | Maximal Rectangle | Math & Logic | Python | O(n) | O(1) |
| 115 | Distinct Subsequences | Dynamic Programming | JavaScript | O(n) | O(n) |
| 154 | Find Minimum in Rotated Sorted Array II | Binary Search | C++ | O(n) | O(1) |
| 185 | Department Top Three Salaries | Database / SQL | SQL | O(n) | O(n) |
| 262 | Trips and Users | Database / SQL | SQL | O(n) | O(n) |
| 301 | Remove Invalid Parentheses | Stack & Queue | Python | O(n) | O(n) |
| 601 | Human Traffic of Stadium | Database / SQL | SQL | O(n) | O(n) |
| 940 | Distinct Subsequences II | Dynamic Programming | JavaScript | O(n) | O(n) |
| 1096 | Brace Expansion II | Array / General | JavaScript | O(n) | O(1) |
| 1301 | Number of Paths with Max Score | Dynamic Programming | PHP | O(n) | O(n) |
| 1340 | Jump Game V | Dynamic Programming | C++ | O(n) | O(n) |
| 1345 | Jump Game IV | Hash Map | C++ | O(n) | O(n) |
| 1406 | Stone Game III | Dynamic Programming | PHP | O(n) | O(n) |
| 1510 | Stone Game IV | Dynamic Programming | TypeScript | O(n) | O(n) |
| 1520 | Maximum Number of Non-Overlapping Substrings | Heap | JavaScript | O(n) | O(n) |
| 1563 | Stone Game V | Prefix Sum | Java | O(n) | O(n) |
| 1665 | Minimum Initial Energy to Finish Tasks | Binary Search | C++ | O(log n) | O(1) |
| 1840 | Maximum Building Height | Tree & Graph | Python | O(n) | O(n) |
| 1872 | Stone Game VIII | Prefix Sum | Java | O(n) | O(n) |
| 2213 | Longest Substring of One Repeating Character | Binary Search | Java | O(n) | O(1) |
| 2267 | Check if There Is a Valid Parentheses String Path | Stack & Queue | C++ | O(n) | O(n) |
| 2267 | Check if There Is a Valid Parentheses String Path | Stack & Queue | C++ | O(n) | O(n) |
| 2472 | Maximum Number of Non-overlapping Palindrome Substrings | Two Pointers | JavaScript | O(n) | O(1) |
| 2630 | Memoize II | Hash Map | TypeScript | O(n) | O(n) |
| 2650 | Design Cancellable Function | Array / General | TypeScript | O(n) | O(1) |
| 3093 | Longest Common Suffix Queries | Hash Map | Python | O(n) | O(n) |
| 3116 | Kth Smallest Amount With Single Denomination Combination | Backtracking | Java | O(n) | O(1) |
| 3161 | Block Placement Queries | Array / General | Python | O(n) | O(1) |
| 3312 | Sorted GCD Pair Queries | Dynamic Programming | PHP | O(n) | O(n) |
| 3336 | Find the Number of Subsequences With Equal GCD | Dynamic Programming | PHP | O(n) | O(n) |
| 3348 | Smallest Divisible Digit Product II | Math & Logic | Python, TypeScript | O(n) | O(1) |
| 3414 | Maximum Score of Non-overlapping Intervals | Heap | JavaScript | O(n) | O(n) |
| 3451 | Find Invalid IP Addresses | Database / SQL | SQL | O(n) | O(n) |
| 3482 | Analyze Organization Hierarchy | Array / General | Python | O(n) | O(1) |
| 3501 | Maximize Active Section with Trade II | Array / General | PHP | O(n) | O(1) |
| 3518 | Smallest Palindromic Rearrangement II | Tree & Graph | PHP | O(n) | O(n) |
| 3525 | Find X Value of Array II | Binary Search | JavaScript | O(n) | O(1) |
| 3534 | Path Existence Queries in a Graph II | Tree & Graph | PHP | O(n) | O(n) |
| 3554 | Find Category Recommendation Pairs | Array / General | Python | O(n) | O(1) |
| 3559 | Number of Ways to Assign Edge Weights II | Tree & Graph | Python | O(n) | O(n) |
| 3614 | Process String with Special Operations II | Tree & Graph | Python | O(n) | O(n) |
| 3617 | Find Students with Study Spiral Pattern | Database / SQL | SQL | O(n) | O(n) |
| 3620 | Network Recovery Pathways | Tree & Graph | PHP | O(n) | O(n) |
| 3673 | Find Zombie Sessions | Database / SQL | SQL | O(n) | O(n) |
| 3691 | Maximum Total Subarray Value II | Array / General | Python | O(n) | O(1) |
| 3699 | Number of ZigZag Arrays I | Hash Map | PHP | O(n) | O(n) |
| 3700 | Number of ZigZag Arrays II | Tree & Graph | PHP | O(n) | O(n) |
| 3734 | Lexicographically Smallest Palindromic Permutation Greater Than Target | Backtracking | Java | O(n) | O(1) |
| 3739 | Count Subarrays With Majority Element II | Array / General | PHP | O(n) | O(1) |
| 3753 | Total Waviness of Numbers in Range II | Array / General | Python | O(n) | O(1) |
| 3764 | Most Common Course Pairs | Database / SQL | SQL | O(n) | O(n) |
| 3832 | Find Users with Persistent Behavior Patterns | Database / SQL | SQL | O(n) | O(n) |