ID	Question	Diff	Freq	Data Structure	Algorithms

	1	Two Sum	2	5	array	sort
					set	Two Pointers
	2	Add Two Numbers	3	4	linked list	Two Pointers
						Math
	3	Longest Substring Without Repeating Characters	3	2	string	Two Pointers
					hashtable
	4	Median of Two Sorted Arrays	5	3	array	Binary Search
	5	Longest Palindromic Substring	4	2	string
	6	ZigZag Conversion	3	1	string
	7	Reverse Integer	2	3		Math
	8	String to Integer (atoi)	2	5	string	Math
	9	Palindrome Number	2	2		Math
	10	Regular Expression Matching	5	3	string	Recursion
						DP
	11	Container With Most Water	3	2	array	Two Pointers
	12	Integer to Roman	3	4		Math
	13	Roman to Integer	2	4		Math
	14	Longest Common Prefix	2	1	string
	15	3Sum	3	5	array	Two Pointers
	16	3Sum Closest	3	1	array	Two Pointers
	17	Letter Combinations of a Phone Number	3	3	string	DFS
	18	4Sum	3	2	array
	19	Remove Nth Node From End of List	2	3	linked list	Two Pointers
	20	Valid Parentheses	2	5	string	Stack
	21	Merge Two Sorted Lists	2	5	linked list	sort
						Two Pointers
						merge
	22	Generate Parentheses	3	4	string	DFS
	23	Merge k Sorted Lists	3	4	linked list	sort
					heap	Two Pointers
						merge
	24	Swap Nodes in Pairs	2	4	linked list
	25	Reverse Nodes in k-Group	4	2	linked list	Recursion
						Two Pointers
	26	Remove Duplicates from Sorted Array	1	3	array	Two Pointers
	27	Remove Element	1	4	array	Two Pointers
	28	Implement strStr()	4	5	string	Two Pointers
						KMP
						rolling hash
	29	Divide Two Integers	4	3		Binary Search
						Math
	30	Substring with Concatenation of All Words	3	1	string	Two Pointers
	31	Next Permutation	5	2	array	permutation
	32	Longest Valid Parentheses	4	1	string	DP
	33	Search in Rotated Sorted Array	4	3	array	Binary Search
	34	Search for a Range	4	3	array	Binary Search
	35	Search Insert Position	2	2	array
	36	Valid Sudoku	2	2	array
	37	Sudoku Solver	4	2	array	DFS
	38	Count and Say	2	2	string	Two Pointers
	39	Combination Sum	3	3	array	combination
	40	Combination Sum II	4	2	array	combination
	41	First Missing Positive	5	2	array	sort
	42	Trapping Rain Water	4	2	array	Two Pointers
						Stack
	43	Multiply Strings	4	3	string	Two Pointers
						Math
	44	Wildcard Matching	5	3	string	Recursion
						DP
						greedy
	45	Jump Game II	4	2	array
	46	Permutations	3	4	array	permutation
	47	Permutations II	4	2	array	permutation
	48	Rotate Image	4	2	array
	49	Anagrams	3	4	string
					hashtable
	50	Pow(x, n)	3	5		Binary Search
						Math
	51	N-Queens	4	3	array	DFS
	52	N-Queens II	4	3	array	DFS
	53	Maximum Subarray	3	3	array	DP
	54	Spiral Matrix	4	2	array
	55	Jump Game	3	2	array
	56	Merge Intervals	4	5	array	sort
					linked list	merge
					red-black tree
	57	Insert Interval	4	5	array	sort
					linked list	merge
					red-black tree
	58	Length of Last Word	1	1	string
	59	Spiral Matrix II	3	2	array
	60	Permutation Sequence	5	1		permutation
						Math
	61	Rotate List	3	2	linked list	Two Pointers
	62	Unique Paths	2	3	array	DP
	63	Unique Paths II	3	3	array	DP
	64	Minimum Path Sum	3	3	array	DP
	65	Valid Number	2	5	string	Math
	66	Plus One	1	2	array	Math
	67	Add Binary	2	4	string	Two Pointers
						Math
	68	Text Justification	4	2	string
	69	Sqrt(x)	4	4		Binary Search
	70	Climbing Stairs	2	5		DP
	71	Simplify Path	3	1	string	Stack
	72	Edit Distance	4	3	string	DP
	73	Set Matrix Zeroes	3	5	array
	74	Search a 2D Matrix	3	3	array	Binary Search
	75	Sort Colors	4	2	array	sort
						Two Pointers
	76	Minimum Window Substring	4	2	string	Two Pointers
	77	Combinations	3	4		combination
	78	Subsets	3	4	array	Recursion
						combination
	79	Word Search	3	4	array	DFS
	80	Remove Duplicates from Sorted Array II	2	2	array	Two Pointers
	81	Search in Rotated Sorted Array II	5	3	array	Binary Search
	82	Remove Duplicates from Sorted List II	3	3	linked list	Recursion
						Two Pointers
	83	Remove Duplicates from Sorted List	1	3	linked list
	84	Largest Rectangle in Histogram	5	2	array	Stack
	85	Maximal Rectangle	5	1	array	DP
						Stack
	86	Partition List	3	3	linked list	Two Pointers
	87	Scramble String	5	2	string	Recursion
						DP
	88	Merge Sorted Array	2	5	array	Two Pointers
						merge
	89	Gray Code	4	2		combination
	90	Subsets II	4	2	array	Recursion
						combination
	91	Decode Ways	3	4	string	Recursion
						DP
	92	Reverse Linked List II	3	2	linked list	Two Pointers
	93	Restore IP Addresses	3	3	string	DFS
	94	Binary Tree Inorder Traversal	4	3	tree	Recursion
					hashtable	morris
						Stack
	95	Unique Binary Search Trees II	4	1	tree	DP
						DFS
	96	Unique Binary Search Trees	3	1	tree	DP
	97	Interleaving String	5	2	string	Recursion
						DP
	98	Validate Binary Search Tree	3	5	tree	DFS
	99	Recover Binary Search Tree	4	2	tree	DFS
	100	Same Tree	1	1	tree	DFS
	101	Symmetric Tree	1	2	tree	DFS
	102	Binary Tree Level Order Traversal	3	4	tree	BFS
	103	Binary Tree Zigzag Level Order Traversal	4	3	queue	BFS
					tree	Stack
	104	Maximum Depth of Binary Tree	1	1	tree	DFS
	105	Construct Binary Tree from Preorder and Inorder Tr	3	3	array	DFS
					tree
	106	Construct Binary Tree from Inorder and Postorder T	3	3	array	DFS
					tree
	107	Binary Tree Level Order Traversal II	3	1	tree	BFS
	108	Convert Sorted Array to Binary Search Tree	2	3	tree	DFS
	109	Convert Sorted List to Binary Search Tree	4	3	linked list	Recursion
						Two Pointers
	110	Balanced Binary Tree	1	2	tree	DFS
	111	Minimum Depth of Binary Tree	1	1	tree	DFS
	112	Path Sum	1	3	tree	DFS
	113	Path Sum II	2	2	tree	DFS
	114	Flatten Binary Tree to Linked List	3	3	tree	Recursion
						Stack
	115	Distinct Subsequences	4	2	string	DP
	116	Populating Next Right Pointers in Each Node	3	3	tree	DFS
	117	Populating Next Right Pointers in Each Node II	4	2	tree	DFS
	118	Pascal's Triangle	2	1	array
	119	Pascal's Triangle II	2	1	array
	120	Triangle	3	1	array	DP
	121	Best Time to Buy and Sell Stock	2	1	array	DP
	122	Best Time to Buy and Sell Stock II	3	1	array	greedy
	123	Best Time to Buy and Sell Stock III	4	1	array	DP
	124	Binary Tree Maximum Path Sum	4	2	tree	DFS
	125	Valid Palindrome	2	5	string	Two Pointers
	126	Word Ladder II	1	1
	127	Word Ladder	3	5	graph	BFS
						shortest path
	128	Longest Consecutive Sequence	4	3	array
	129	Sum Root to Leaf Numbers	2	4	tree	DFS
	130	Surrounded Regions	4	3	array	BFS
						DFS
	131	Palindrome Partitioning	3	4	string	DFS
	132	Palindrome Partitioning II	4	3	string	DP