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LeetCode

This is the list of my accepted LeetCode solutions in Java for more than 150 problems (as of 01/03/2020).

How to Read the List

  • The first column is the index number of each question in LeetCode.
  • The second column is the name of the problem. It is linked to each problem page in LeetCode.
  • The third column shows the problem's difficulty in LeetCode.
  • The forth column has my solutions for each problem. Each solution is linked to my Java code in this repository. The list of abbreviations in this section is provided bellow.
  • The fifth column shows the link to the JUnit test cases for the problem. "-" means I have not created test cases for the problem yet.
  • The last column is the date when I solved the problem last time.

Abbreviations

DP = Dynamic Programming
D&C = Divide and Conquer
DFS = Depth First Search
BFS = Breadth First Search

Note

* = problems that can be improved in time or space complexity.
** = problems that I should revisit in the future because I spent too long on solving it.
*** = problems that have follow-up or different approaches.
^ = problems that I like.

Log

  • Solved 150 problems (as of 01/03/2020).
  • Solved 100 problems (as of 10/29/2019).

Java Solution List

# Problem Name Difficulty Solutions Test Case Last Solved
1 Two Sum Easy Brute Force / Hash Table array 05/21/2020
2 Add Two Numbers*** Medium Recursion linkedlist 08/18/2019
3 Longest Substring Without Repeating Characters*** Medium Iteration & Set / Sliding Window (Opt) & Table string 05/06/2020
5 Longest Palindromic Substring* Medium Expand Around Center string 10/05/2019
9 Palindrome Number* Easy Recursion - 08/18/2019
11 Container With Most Water** Medium Two Pointers array 11/18/2019
12 Integer to Roman* Medium Iteration integer 05/05/2020
13 Roman to Integer* Easy Recursion string 06/17/2019
14 Longest Common Prefix*** Easy Vertical Scanning / Horizontal Scanning array 05/29/2020
17 Letter Combinations of a Phone Number* Medium Recursion string 11/07/2019
19 Remove Nth Node From End of List**^ Medium Recursion linkedlist 08/20/2019
20 Valid Parentheses* Easy Recursion / Stack / Stack & Map string 05/05/2020
21 Merge Two Sorted Lists** Easy Recursion linkedlist 08/20/2019
22 Generate Parentheses* Medium DP (Bottom up) integer 11/28/2019
23 Merge k Sorted Lists*** Hard D&C linkedlist 08/27/2019
24 Swap Nodes in Pairs*** Medium Recursion linkedlist 05/12/2020
26 Remove Duplicates from Sorted Array** Easy Two Pointers / For Each array 05/19/2020
27 Remove Element*** Easy Two Pointers array 06/04/2020
28 Implement strStr()* Easy Brute Force string 05/25/2020
31 Next Permutation* Medium Swap and Reverse array 11/28/2019
32 Longest Valid Parentheses* Hard Stack string 12/12/2019
33 Search in Rotated Sorted Array*** Medium Recursion & BS array 11/16/2019
34 Find First and Last Position of Element in Sorted Array* Medium Binary Search & Iteration array 12/02/2019
35 Search Insert Position* Easy Binary Search & Iteration array 05/04/2020
36 Valid Sudoku*** Medium Hash Table matrix 05/22/2020
38 Count and Say Easy DP (Bottom up) integer 05/29/2020
39 Combination Sum* Medium Backtracking array 08/29/2019
41 First Missing Positive***^ Hard Mark as Negatives array 11/22/2019
46 Permutations*** Medium Backtracking array 10/13/2019
47 Permutations II* Medium Backtracking & String Set array 06/21/2020
48 Rotate Image*** Medium Layer by Layer / Two Reverses matrix 05/24/2020
49 Group Anagrams* Medium Hash Map & Sort array 06/03/2020
53 Maximum Subarray*** Easy DP (Bottom up) array 05/30/2020
54 Spiral Matrix** Medium Recursion matrix 08/22/2019
55 Jump Game**^ Medium Greedy array 08/23/2019
56 Merge Intervals** Medium Recursion interval 08/13/2019
57 Insert Interval* Hard Iteration interval 12/11/2019
61 Rotate List* Medium Recursion linkedlist 06/19/2020
62 Unique Paths** Medium -
64 Minimum Path Sum* Medium A* matrix 11/29/2019
66 Plus One*** Easy Iteration array 05/21/2020
70 Climbing Stairs Easy DP (Bottom up) integer 08/14/2019
73 Set Matrix Zeroes*** Medium O(M+N) space matrix 08/13/2019
75 Sort Colors* Medium Iteration Two Pass array 10/31/2019
76 Minimum Window Substring* Hard Sliding Window string 09/24/2019
77 Combinations*** Medium Backtracking / Backtracking (Opt) integer 01/12/2020
78 Subsets*** Medium Iteration array 10/10/2019
79 Word Search* Medium DFS matrix 08/22/2019
80 Remove Duplicates from Sorted Array II* Medium Iteration array 05/14/2020
88 Merge Sorted Array* Easy Iteration array 11/03/2019
91 Decode Ways* Medium D&C string 09/28/2019
92 Reverse Linked List II** Medium Recursion linkedlist 08/18/2019
94 Binary Tree Inorder Traversal*** Medium Recursion binarytree 08/02/2019
96 Unique Binary Search Trees*** Medium DP (Top down) integer 01/13/2020
98 Validate Binary Search Tree* Medium Inorder binarytree 08/18/2019
99 Recover Binary Search Tree* Hard Inorder & List binarytree 05/09/2020
100 Same Tree Easy Recursion binarytree 08/13/2019
101 Symmetric Tree** Easy Recursion binarytree 11/04/2019
102 Binary Tree Level Order Traversal*** Medium Recursion binarytree 08/18/2019
103 Binary Tree Zigzag Level Order Traversal** Medium BFS and Reverse binarytree 11/11/2019
104 Maximum Depth of Binary Tree Easy Recursion / Queue & Iteration binarytree 12/26/2019
105 Construct Binary Tree from Preorder and Inorder Traversal** Medium Recursion / Recursion (Opt) binarytree 10/17/2019
107 Binary Tree Level Order Traversal II*** Easy DFS & Recursion binarytree 07/10/2020
108 Convert Sorted Array to Binary Search Tree^ Easy Recursion binarytree 06/17/2019
110 Balanced Binary Tree*** Easy Recursion binarytree 06/26/2020
112 Path Sum* Easy DFS & Recursion binarytree 07/05/2020
114 Flatten Binary Tree to Linked List** Medium Recursion binarytree 12/05/2019
116 Populating Next Right Pointers in Each Node* Medium Stack - 11/16/2019
118 Pascal's Triangle*** Easy DP (Bottom up) - 10/29/2019
120 Triangle* Medium DP (Bottom up) array 06/29/2020
121 Best Time to Buy and Sell Stock* Easy Iteration Two Pass array 10/15/2019
122 Best Time to Buy and Sell Stock II* Easy Iteration (Peak Valley) / Iteration (One Pass) array 05/19/2020
124 Binary Tree Maximum Path Sum*** Hard Recursion binarytree 10/07/2019
125 Valid Palindrome*** Easy 2 Pointers / 2 Pointers (Opt) string 05/08/2020
127 Word Ladder* Medium BFS string 11/09/2019
128 Longest Consecutive Sequence* Hard Hash Map with Class array 09/23/2019
130 Surrounded Regions** Medium DFS From Edges matrix 12/17/2019
133 Clone Graph Medium -
136 Single Number* Easy Set / Set & Math array 05/20/2020
138 Copy List with Random Pointer* Medium Recursion & Map - 11/23/2019
141 Linked List Cycle*** Easy Recursion / Floyd's Cycle Detection linkedlist 10/01/2019
142 Linked List Cycle II** Medium Floyd's Cycle Detection linkedlist 12/02/2019
143 Reorder List* Medium Recursion linkedlist 08/22/2019
144 Binary Tree Preorder Traversal*** Medium Recursion binarytree 01/26/2020
145 Binary Tree Postorder Traversal*** Hard Recursion binarytree 06/11/2019
148 Sort List* Medium Quick Sort linkedlist 12/24/2019
150 Evaluate Reverse Polish Notation*** Medium Stack array 12/16/2019
152 Maximum Product Subarray** Medium D&C array 09/27/2019
153 Find Minimum in Rotated Sorted Array*** Medium Recursion array 08/28/2019
155 Min Stack*** Easy Two Node Pointers - 12/04/2019
160 Intersection of Two Linked Lists*** Easy Recursion with Count linkedlist 12/03/2019
162 Find Peak Element** Medium Recursive Binary Search array 11/12/2019
169 Majority Element Easy -
171 Excel Sheet Column Number*** Easy Iteration string 10/08/2019
173 Binary Search Tree Iterator*** Medium Stack binarytree 01/29/2020
189 Rotate Array*** Easy 3 Reverses array 05/19/2020
198 House Robber Easy DP (Bottom up) array 07/31/2019
199 Binary Tree Right Side View* Medium BFS & Queue binarytree 07/03/2020
200 Number of Islands* Medium DFS matrix 10/04/2019
202 Happy Number* Easy Hash Set integer 11/10/2019
206 Reverse Linked List Easy Iteration / Recursion linkedlist 08/17/2019
207 Course Schedule** Medium DFS graph 08/15/2019
208 Implement Trie (Prefix Tree)*** Medium Recursion string 10/18/2019
210 Course Schedule II* Medium DFS graph 12/19/2019
211 Add and Search Word - Data structure design* Medium Trie string 10/03/2019
212 Word Search II* Hard DFS matrix 10/06/2019
213 House Robber II*** Medium DP (Bottom up) array 10/16/2019
215 Kth Largest Element in an Array*** Medium Sort array 10/27/2019
217 Contains Duplicate*** Easy Hash Set1 / Hash Set2 array 05/20/2020
226 Invert Binary Tree*** Easy Recursion binarytree 08/18/2019
227 Basic Calculator II* Medium Two Lists string 11/05/2019
230 Kth Smallest Element in a BST*** Medium Recursion binarytree 08/27/2019
234 Palindrome Linked List* Easy Recursion linkedlist 12/04/2019
235 Lowest Common Ancestor of a Binary Search Tree*** Easy Recursion binarytree 08/28/2019
237 Delete Node in a Linked List Easy Swap linkedlist 01/06/2020
238 Product of Array Except Self* Medium Left And Right Product Lists array 02/16/2020
242 Valid Anagram* Easy Sort / Hash Table string 05/24/2020
260 Single Number III* Medium -
268 Missing Number*** Easy Formula array 08/29/2019
278 First Bad Version*** Easy Binary Search integer 05/30/2020
279 Perfect Squares* Medium DP (Bottom up) integer 11/06/2019
283 Move Zeroes Easy Two Pointers / Iteration array 05/21/2020
287 Find the Duplicate Number*^ Medium Brute Force / Floyd's Cycle Detection array 10/01/2019
289 Game of Life*** Medium In-place (Opt) matrix 12/18/2019
295 Find Median from Data Stream* Hard Sort / Insertion Sort array 10/09/2019
297 Serialize and Deserialize Binary Tree* Hard Iteration binarytree 09/27/2019
300 Longest Increasing Subsequence* Medium DP (Bottom up) array 09/26/2019
315 Count of Smaller Numbers After Self*** Hard Binary Search & Iteration array 12/09/2019
322 Coin Change Medium DP (Bottom up) array 08/15/2019
328 Odd Even Linked List Medium Iteration linkedlist 08/02/2019
329 Longest Increasing Path in a Matrix*** Hard DFS matrix 12/06/2019
334 Increasing Triplet Subsequence** Medium Iteration array 01/03/2020
337 House Robber III Medium Recursion binarytree 11/30/2019
341 Flatten Nested List Iterator* Medium Stack - 12/18/2019
344 Reverse String Easy Iteration / Recursion string 05/23/2018
347 Top K Frequent Elements*** Medium Map array 08/27/2019
349 Intersection of Two Arrays Easy Two Sets array 08/19/2019
350 Intersection of Two Arrays II* Easy Map & Iteration / 2 Maps array 05/20/2020
373 Find K Pairs with Smallest Sums* Medium Dijkstra array 06/23/2020
387 First Unique Character in a String*** Easy Hash Table string 05/24/2020
389 Find the Difference* Easy Array string 02/21/2020
394 Decode String*** Medium Stack string 12/11/2019
412 Fizz Buzz* Easy Iteration integer 06/19/2020
417 Pacific Atlantic Water Flow** Medium DFS matrix 10/02/2019
424 Longest Repeating Character Replacement* Medium Sliding Window string 10/20/2019
435 Non-overlapping Intervals* Medium Greedy interval 10/14/2019
437 Path Sum III* Easy Recursion & List binarytree 11/30/2019
438 Find All Anagrams in a String* Medium Map & Iteration string 12/04/2019
445 Add Two Numbers II*** Medium Recursion linkedlist 06/14/2019
448 Find All Numbers Disappeared in an Array** Easy Mark as Negatives array 11/17/2019
461 Hamming Distance* Easy -
508 Most Frequent Subtree Sum* Medium Recursion binarytree 06/12/2019
509 Fibonacci Number Easy -
543 Diameter of Binary Tree*** Easy Recursion binarytree 11/20/2019
547 Friend Circles* Medium -
560 Subarray Sum Equals K* Medium Brute Force array 12/24/2019
572 Subtree of Another Tree*** Easy Recursion binarytree 08/29/2019
581 Shortest Unsorted Continuous Subarray*** Easy Iteration & Two Pass array 11/26/2019
599 Minimum Index Sum of Two Lists* Easy 2 Maps array 01/11/2020
617 Merge Two Binary Trees*** Easy Recursion binarytree 11/14/2019
623 Add One Row to Tree*** Medium Recursion binarytree 01/15/2020
647 Palindromic Substrings* Medium DP (Bottom up) string 10/11/2019
654 Maximum Binary Tree* Medium Recursion binarytree 09/30/2019
657 Robot Return to Origin Easy Iteration / Recursion string 08/18/2019
700 Search in a Binary Search Tree Easy Recursion binarytree 06/10/2019
701 Insert into a Binary Search Tree*** Medium Recursion binarytree 05/07/2020
814 Binary Tree Pruning*** Medium Recursion binarytree 12/20/2019
876 Middle of the Linked List*** Easy Iteration linkedlist 04/14/2019
881 Boats to Save People* Medium Sort and Greedy array 05/22/2020
905 Sort Array By Parity*** Easy Two Pointers array 04/20/2019
912 Sort An Array* Medium -
917 Reverse Only Letters* Easy Two Pointers & Iteration string 02/09/2020
938 Range Sum of BST*** Easy Recursion binarytree 10/19/2019
958 Check Completeness of a Binary Tree* Medium Recursion binarytree 01/21/2020
977 Squares of a Sorted Array* Easy Iteration array 12/19/2019
980 Unique Paths III* Hard -
1003 Check If Word Is Valid After Substitutions* Medium Recursion / Stack & Iteration string 01/01/2020
1047 Remove All Adjacent Duplicates In String* Easy Recursion string 06/12/2019
1053 Previous Permutation With One Swap* Medium -
1078 Occurrences After Bigram Easy Iteration string 06/15/2019
1110 Delete Nodes And Return Forest*** Medium Recursion binarytree 01/09/2020
1161 Maximum Level Sum of a Binary Tree*** Medium Recursion binarytree 01/24/2020
1207 Unique Number of Occurrences* Easy Map array 10/24/2019
1222 Queens That Can Attack the King Medium Set & Iteration matrix 05/16/2020
1249 Minimum Remove to Make Valid Parentheses* Medium Stack and Array string 02/22/2020
1261 Find Elements in a Contaminated Binary Tree*** Medium Recursion binarytree 01/25/2020
1295 Find Numbers with Even Number of Digits* Easy Iteration array 01/05/2019
1448 Count Good Nodes in Binary Tree*** Medium Recursion binarytree 05/22/2020