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