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Solutions, unit tests, and code skeletons for problems fromLeetcode OJ. (in progress)
root |--- doc | |-- category.md // category for problems in leetcode | |-- problems.tsv // searchable tsv file listing problems from leetcode | |-- road.md // general things I learnt through the process |--- lib | |-- *.jar // jar file referenced in this project |--- src // solutions for problems | |-- _001_TwoSum | | |-- Practice.java // code skeleton | | |-- Solution.java | | |-- Solution*.java // other solutions for this problem | |-- ...... |--- test // unit tests | |-- _001_TwoSum | | |-- PracticeTest.java | | |-- SolutionTest.java | | |-- Solution*Test.java | |-- ......- Discussions and code reviews are more than welcome.
Solution.javaprovides OJ-accepted solution.Practice.javaprovides code skeleton for each problemSolutionTest.javaandPracticeTest.javaprovides basic unit tests to verify your solution.
P.S.:
- Please use Java 8 to compile and run
- Many solutions are from online sources, like Leetcode OJ discussion, coders' blogs, etc.
- Solution that passed unit tests does not guarantee to pass Leetcode OJ.
| # | Problem | Difficulty | Tags | Note |
|---|---|---|---|---|
| 001 | Two Sum | Medium | ArrayHash Table | O(N^2) -> O(N) + O(N) hashtable |
| 002 | Add Two Numbers | Medium | Linked ListMath | |
| 003 | Longest Substring Without Repeating Characters | Medium | Hash TableTwo PointersString | two pointers to keep window -> O(N) |
| 004 | Median of Two Sorted Arrays | Hard | Divide and ConquerArrayBinary Search | |
| 005 | Longest Palindromic Substring | Medium | String | expanding two pointers |
| 006 | ZigZag Conversion | Easy | String | |
| 007 | Reverse Integer | Easy | Math | How to monitor Stack Overflow |
| 008 | String to Integer (atoi) | Easy | MathString | |
| 009 | Palindrome Number | Easy | Math | 1) count digits; 2) get digit from number's front/back; 3) to avoid StackOverflow, only reverse half part of number |
| 010 | Regular Expression Matching | Hard | Dynamic ProgrammingBacktrackingString | finite state machine! |
| 011 | Container With Most Water | Medium | ArrayTwo Pointers | narrowing two pointers |
| 012 | Integer to Roman | Medium | MathString | |
| 013 | Roman to Integer | Easy | MathString | |
| 014 | Longest Common Prefix | Easy | String | |
| 015 | 3Sum | Medium | ArrayTwo Pointers | sorting then 2sum |
| 016 | 3Sum Closest | Medium | ArrayTwo Pointers | |
| 017 | Letter Combinations of a Phone Number | Medium | BacktrackingString | BFS: iterative V.S. DFS: backtracking |
| 018 | 4Sum | Medium | ArrayHash TableTwo Pointers | |
| 019 | Remove Nth Node From End of List | Easy | Linked ListTwo Pointers | fast-slow two pointers on Linked List |
| 020 | Valid Parentheses | Easy | StackString | stack basic, corner case |
| 021 | Merge Two Sorted Lists | Easy | Linked List | |
| 022 | Generate Parentheses | Medium | BacktrackingString | BT template |
| 023 | Merge k Sorted Lists | Hard | Divide and ConquerLinked ListHeap | |
| 024 | Swap Nodes in Pairs | Medium | Linked List | |
| 025 | Reverse Nodes in k-Group | Hard | Linked List | |
| 026 | Remove Duplicates from Sorted Array | Easy | ArrayTwo Pointers | partition techinque on array |
| 027 | Remove Element | Easy | ArrayTwo Pointers | |
| 028 | Implement strStr() | Easy | Two PointersString | |
| 029 | Divide Two Integers | Medium | MathBinary Search | ! |
| 030 | Substring with Concatenation of All Words | Hard | Hash TableTwo PointersString | |
| 031 | Next Permutation | Medium | Array | |
| 032 | Longest Valid Parentheses | Hard | Dynamic ProgrammingString | DP |
| 033 | Search in Rotated Sorted Array | Hard | ArrayBinary Search | |
| 034 | Search for a Range | Medium | ArrayBinary Search | ! one binary search to find start position, and another one find ending position |
| 035 | Search Insert Position | Medium | ArrayBinary Search | standard binary search with minor modification |
| 036 | Valid Sudoku | Easy | Hash Table | |
| 037 | Sudoku Solver | Hard | BacktrackingHash Table | |
| 038 | Count and Say | Easy | String | |
| 039 | Combination Sum | Medium | ArrayBacktracking | BT template + prune siblings |
| 040 | Combination Sum II | Medium | ArrayBacktracking | BT template + prune siblings + skip duplicate |
| 041 | First Missing Positive | Hard | Array | swap-until technique |
| 042 | Trapping Rain Water | Hard | ArrayStackTwo Pointers | !! |
| 043 | Multiply Strings | Medium | MathString | ! simulate multiplication |
| 044 | Wildcard Matching | Hard | Dynamic ProgrammingBacktrackingGreedyString | |
| 045 | Jump Game II | Hard | ArrayGreedy | |
| 046 | Permutations | Medium | Backtracking | backtrack template + select to add |
| 047 | Permutations II | Hard | Backtracking | backtrack template + select to add + skip duplicate using set |
| 048 | Rotate Image | Medium | Array | |
| 049 | Group Anagrams | Medium | Hash TableString | !! |
| 050 | Pow(x, n) | Medium | MathBinary Search | |
| 051 | N-Queens | Hard | Backtracking | |
| 052 | N-Queens II | Hard | Backtracking | |
| 053 | Maximum Subarray | Medium | Divide and ConquerArrayDynamic Programming | |
| 054 | Spiral Matrix | Medium | Array | |
| 055 | Jump Game | Medium | ArrayGreedy | |
| 056 | Merge Intervals | Hard | ArraySort | how to decide the boundary of merged interval |
| 057 | Insert Interval | Hard | ArraySort | ! enlarge to merge interval |
| 058 | Length of Last Word | Easy | String | |
| 059 | Spiral Matrix II | Medium | Array | |
| 060 | Permutation Sequence | Medium | BacktrackingMath | |
| 061 | Rotate List | Medium | Linked ListTwo Pointers | fast-slow pointers; [189] Rotate Array |
| 062 | Unique Paths | Medium | ArrayDynamic Programming | |
| 063 | Unique Paths II | Medium | ArrayDynamic Programming | |
| 064 | Minimum Path Sum | Medium | ArrayDynamic Programming | |
| 065 | Valid Number | Hard | MathString | |
| 066 | Plus One | Easy | ArrayMath | |
| 067 | Add Binary | Easy | MathString | add binary numbers |
| 068 | Text Justification | Hard | String | ! very careful string operations |
| 069 | Sqrt(x) | Medium | MathBinary Search | !binary search along y-axle |
| 070 | Climbing Stairs | Easy | Dynamic Programming | |
| 071 | Simplify Path | Medium | StackString | |
| 072 | Edit Distance | Hard | Dynamic ProgrammingString | |
| 073 | Set Matrix Zeroes | Medium | Array | |
| 074 | Search a 2D Matrix | Medium | ArrayBinary Search | 2D coordidates --> 1D index |
| 075 | Sort Colors | Medium | ArrayTwo PointersSort | |
| 076 | Minimum Window Substring | Hard | Hash TableTwo PointersString | ![substring/sublist] hashing + two pointers -> O(N) |
| 077 | Combinations | Medium | Backtracking | |
| 078 | Subsets | Medium | ArrayBacktrackingBit Manipulation | seeREADME |
| 079 | Word Search | Medium | ArrayBacktracking | |
| 080 | Remove Duplicates from Sorted Array II | Medium | ArrayTwo Pointers | from 2 to k (general) |
| 081 | Search in Rotated Sorted Array II | Medium | ArrayBinary Search | |
| 082 | Remove Duplicates from Sorted List II | Medium | Linked List | |
| 083 | Remove Duplicates from Sorted List | Easy | Linked List | |
| 084 | Largest Rectangle in Histogram | Hard | ArrayStack | |
| 085 | Maximal Rectangle | Hard | ArrayHash TableStackDynamic Programming | |
| 086 | Partition List | Medium | Linked ListTwo Pointers | |
| 087 | Scramble String | Hard | Dynamic ProgrammingString | |
| 088 | Merge Sorted Array | Easy | ArrayTwo Pointers | in-place merging -> from end to begin; two pointers on two lists |
| 089 | Gray Code | Medium | Backtracking | |
| 090 | Subsets II | Medium | ArrayBacktracking | backtrack template + skip duplicate |
| 091 | Decode Ways | Medium | Dynamic ProgrammingString | |
| 092 | Reverse Linked List II | Medium | Linked List | |
| 093 | Restore IP Addresses | Medium | BacktrackingString | |
| 094 | Binary Tree Inorder Traversal | Medium | TreeHash TableStack | |
| 095 | Unique Binary Search Trees II | Medium | TreeDynamic Programming | |
| 096 | Unique Binary Search Trees | Medium | TreeDynamic Programming | |
| 097 | Interleaving String | Hard | Dynamic ProgrammingString | |
| 098 | Validate Binary Search Tree | Medium | TreeDepth-first Search | |
| 099 | Recover Binary Search Tree | Hard | TreeDepth-first Search | |
| 100 | Same Tree | Easy | TreeDepth-first Search | |
| 101 | Symmetric Tree | Easy | TreeDepth-first Search | |
| 102 | Binary Tree Level Order Traversal | Easy | TreeBreadth-first Search | |
| 103 | Binary Tree Zigzag Level Order Traversal | Medium | TreeBreadth-first SearchStack | |
| 104 | Maximum Depth of Binary Tree | Easy | TreeDepth-first Search | |
| 105 | Construct Binary Tree from Preorder and Inorder Traversal | Medium | TreeArrayDepth-first Search | |
| 106 | Construct Binary Tree from Inorder and Postorder Traversal | Medium | TreeArrayDepth-first Search | |
| 107 | Binary Tree Level Order Traversal II | Easy | TreeBreadth-first Search | |
| 108 | Convert Sorted Array to Binary Search Tree | Medium | TreeDepth-first Search | |
| 109 | Convert Sorted List to Binary Search Tree | Medium | Depth-first SearchLinked List | |
| 110 | Balanced Binary Tree | Easy | TreeDepth-first Search | |
| 111 | Minimum Depth of Binary Tree | Easy | TreeDepth-first SearchBreadth-first Search | |
| 112 | Path Sum | Easy | TreeDepth-first Search | |
| 113 | Path Sum II | Medium | TreeDepth-first Search | |
| 114 | Flatten Binary Tree to Linked List | Medium | TreeDepth-first Search | |
| 115 | Distinct Subsequences | Hard | Dynamic ProgrammingString | |
| 116 | Populating Next Right Pointers in Each Node | Medium | TreeDepth-first Search | |
| 117 | Populating Next Right Pointers in Each Node II | Hard | TreeDepth-first Search | |
| 118 | Pascal's Triangle | Easy | Array | |
| 119 | Pascal's Triangle II | Easy | Array | |
| 120 | Triangle | Medium | ArrayDynamic Programming | |
| 121 | Best Time to Buy and Sell Stock | Medium | ArrayDynamic Programming | O(N^2) -> O(N) |
| 122 | Best Time to Buy and Sell Stock II | Medium | ArrayGreedy | |
| 123 | Best Time to Buy and Sell Stock III | Hard | ArrayDynamic Programming | |
| 124 | Binary Tree Maximum Path Sum | Hard | TreeDepth-first Search | post-order, local and global ,README |
| 125 | Valid Palindrome | Easy | Two PointersString | |
| 126 | Word Ladder II | Hard | ArrayBacktrackingBreadth-first SearchString | |
| 127 | Word Ladder | Medium | Breadth-first Search | |
| 128 | Longest Consecutive Sequence | Hard | Array | |
| 129 | Sum Root to Leaf Numbers | Medium | TreeDepth-first Search | |
| 130 | Surrounded Regions | Medium | Breadth-first Search | |
| 131 | Palindrome Partitioning | Medium | Backtracking | |
| 132 | Palindrome Partitioning II | Hard | Dynamic Programming | |
| 133 | Clone Graph | Medium | Depth-first SearchBreadth-first SearchGraph | DFS V.S.BFS |
| 134 | Gas Station | Medium | Greedy | |
| 135 | Candy | Hard | Greedy | |
| 136 | Single Number | Medium | Hash TableBit Manipulation | |
| 137 | Single Number II | Medium | Bit Manipulation | |
| 138 | Copy List with Random Pointer | Hard | Hash TableLinked List | |
| 139 | Word Break | Medium | Dynamic Programming | |
| 140 | Word Break II | Hard | Dynamic ProgrammingBacktracking | |
| 141 | Linked List Cycle | Medium | Linked ListTwo Pointers | |
| 142 | Linked List Cycle II | Medium | Linked ListTwo Pointers | |
| 143 | Reorder List | Medium | Linked List | |
| 144 | Binary Tree Preorder Traversal | Medium | TreeStack | |
| 145 | Binary Tree Postorder Traversal | Hard | TreeStack | |
| 146 | LRU Cache | Hard | Data Structure | |
| 147 | Insertion Sort List | Medium | Linked ListSort | |
| 148 | Sort List | Medium | Linked ListSort | ! fast-slow to find middle + iterative reverse + merge sort |
| 149 | Max Points on a Line | Hard | Hash TableMath | |
| 150 | Evaluate Reverse Polish Notation | Medium | Stack | |
| 151 | Reverse Words in a String | Medium | String | README |
| 152 | Maximum Product Subarray | Medium | ArrayDynamic Programming | |
| 153 | Find Minimum in Rotated Sorted Array | Medium | ArrayBinary Search | |
| 154 | Find Minimum in Rotated Sorted Array II | Hard | ArrayBinary Search | |
| 155 | Min Stack | Easy | StackData Structure | |
| 157 | Read N Characters Given Read4 | Easy | String | |
| 158 | Read N Characters Given Read4 II | Hard | String | |
| 160 | Intersection of Two Linked Lists | Easy | Linked List | two pointers on two lists; combine long and short lists into one |
| 161 | One Edit Distance | Medium | String | |
| 162 | Find Peak Element | Medium | ArrayBinary Search | |
| 163 | Missing Ranges | Medium | Array | |
| 164 | Maximum Gap | Hard | Sort | |
| 165 | Compare Version Numbers | Easy | String | |
| 166 | Fraction to Recurring Decimal | Medium | Hash TableMath | |
| 168 | Excel Sheet Column Title | Easy | Math | convert decimal to other BASE system |
| 169 | Majority Element | Easy | Divide and ConquerArrayBit Manipulation | |
| 171 | Excel Sheet Column Number | Easy | Math | convert other BASE system to decimal system |
| 172 | Factorial Trailing Zeroes | Easy | Math | count the number of certain number (e.g. 5) in n! |
| 173 | Binary Search Tree Iterator | Medium | TreeStack | |
| 174 | Dungeon Game | Hard | Dynamic ProgrammingBinary Search | |
| 179 | Largest Number | Medium | Sort | |
| 186 | Reverse words in a string II | Medium | Array | two pass + include last position trick |
| 187 | Repeated DNA Sequences | Medium | Hash TableBit Manipulation | |
| 188 | Best Time to Buy and Sell Stock IV | Hard | Dynamic Programming | |
| 189 | Rotate Array | Easy | Array | |
| 190 | Reverse Bits | Easy | Bit Manipulation | |
| 191 | Number of 1 Bits | Easy | Bit Manipulation | |
| 198 | House Robber | Easy | Dynamic Programming | |
| 199 | Binary Tree Right Side View | Medium | TreeDepth-first SearchBreadth-first Search | |
| 200 | Number of Islands | Medium | Depth-first SearchBreadth-first Search | |
| 201 | Bitwise AND of Numbers Range | Medium | Bit Manipulation | |
| 202 | Happy Number | Easy | Hash TableMath | |
| 203 | Remove Linked List Elements | Easy | Linked List | |
| 204 | Count Primes | Easy | Hash TableMath | sieve algorithm |
| 205 | Isomorphic Strings | Easy | Hash Table | |
| 206 | Reverse Linked List | Easy | Linked List | |
| 207 | Course Schedule | Medium | Depth-first SearchBreadth-first SearchGraphTopological Sort | |
| 208 | Implement Trie (Prefix Tree) | Medium | Data StructureTrie | |
| 209 | Minimum Size Subarray Sum | Medium | ArrayTwo PointersBinary Search | |
| 210 | Course Schedule II | Medium | Depth-first SearchBreadth-first SearchGraphTopological Sort | |
| 211 | Add and Search Word - Data structure design | Medium | BacktrackingData StructureTrie | |
| 212 | Word Search II | Hard | BacktrackingTrie | |
| 213 | House Robber II | Medium | Dynamic Programming | |
| 214 | Shortest Palindrome | Hard | String | |
| 215 | Kth Largest Element in an Array | Medium | Divide and ConquerHeap | max-heap using Java's Priority Queue |
| 216 | Combination Sum III | Medium | ArrayBacktracking | |
| 217 | Contains Duplicate | Easy | ArrayHash Table | |
| 218 | The Skyline Problem | Hard | Divide and ConquerHeap | |
| 219 | Contains Duplicate II | Easy | ArrayHash Table | |
| 220 | Contains Duplicate III | Medium | Binary Search Tree | |
| 221 | Maximal Square | Medium | Dynamic Programming | |
| 222 | Count Complete Tree Nodes | Medium | TreeBinary Search | |
| 223 | Rectangle Area | Easy | Math | |
| 224 | Basic Calculator | Medium | StackMath | |
| 225 | Implement Stack using Queues | Easy | StackData Structure | |
| 226 | Invert Binary Tree | Easy | Tree | |
| 227 | Basic Calculator II | Medium | String | |
| 228 | Summary Ranges | Easy | Array | |
| 229 | Majority Element II | Medium | Array | !vote algorithm |
| 230 | Kth Smallest Element in a BST | Medium | TreeBinary Search | |
| 231 | Power of Two | Easy | MathBit Manipulation | |
| 232 | Implement Queue using Stacks | Easy | StackData Structure | README |
| 233 | Number of Digit One | Medium | Math | |
| 234 | Palindrome Linked List | Easy | Linked ListTwo Pointers | |
| 235 | Lowest Common Ancestor of a Binary Search Tree | Easy | Tree | |
| 236 | Lowest Common Ancestor of a Binary Tree | Medium | Tree | README |
| 237 | Delete Node in a Linked List | Easy | Linked List | |
| 238 | Product of Array Except Self | Medium | Array | O(N^2) -> two pass O(N) |
| 239 | Sliding Window Maximum | Hard | Heap | descending queue |
| 240 | Search a 2D Matrix II | Medium | Divide and ConquerBinary Search | |
| 241 | Different Ways to Add Parentheses | Medium | Divide and Conquer | |
| 242 | Valid Anagram | Easy | Hash TableSort | |
| 246 | Strobogrammatic Number | Easy | Hash TableMath | |
| 251 | Flatten2DVector | Medium | Design | |
| 252 | Meeting Rooms | Easy | Sort | |
| 253 | Meeting Rooms II | Medium | HeapGreedySort | |
| 256 | Paint House | Medium | Dynamic Programming | |
| 257 | Binary Tree Paths | Easy | Tree | |
| 258 | Add Digits | Easy | Math | |
| 260 | Single Number III | Medium | Bit Manipulation | |
| 261 | Graph Valid Tree | Medium | Breadth-first SearchDepth-first SearchGraphUnion-Find | |
| 263 | Ugly Number | Easy | Math | |
| 264 | Ugly Number II | Medium | MathHeap | |
| 265 | Paint House | Hard | Dynamic Programming | |
| 268 | Missing Number | Medium | ArrayMathBit Manipulation | |
| 269 | Alien Dictionary | Hard | GraphTopological Sort | BFS |
| 273 | Integer to English Words | Medium | MathString | |
| 274 | H-Index | Medium | Hash TableSort | |
| 275 | H-Index II | Medium | Binary Search | |
| 278 | First Bad Version | Easy | Binary Search | |
| 279 | Perfect Squares | Medium | Dynamic ProgrammingBreadth-first SearchMath | |
| 282 | Expression Add Operators | Hard | Divide and Conquer | |
| 283 | Move Zeroes | Easy | ArrayTwo Pointers | |
| 284 | Peeking Iterator | Medium | Design | |
| 285 | Inorder Successor in BST | Medium | Tree | |
| 286 | Walls And Gates | Medium | BFS | |
| 287 | Find the Duplicate Number | Hard | ArrayTwo PointersBinary Search | |
| 289 | Game of Life | Medium | Array | |
| 290 | Word Pattern | Easy | Hash Table |
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Leetcode solutions, code skeletons, and unit tests in Java (in progress)
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