No.	Problem	Stumped	Algorithms	time complexity
001	Two Sum	Easy*****	HashMap	O(N)
002	Add Two Numbers	Medium****	LinkedList	O(max(M,N))
008	String to Integer (atoi)	Easy*****	-	O(N)
012	Integer to Roman	Medium****	divide	O(1)
013	Roman to Integer	Easy****	-	O(N)
015	3Sum	Medium*****	traverse	O(N2)
016	3Sum Closest	Medium*****	traverse	O(NlogN)
020	Valid Parentheses	Easy*****	stack	O(N)
021	Merge Two Sorted Lists	Easy*****	recursion	O(N)
022	Generate Parentheses	Medium****	DFS Stack	-
023	Merge k Sorted Lists	Hard****	divide	-
024	Swap Nodes in Pairs	Medium****	recursive	O(N)
027	Remove Element	Easy****	-	O(N)
028	Implement strStr()	Easy*****	-	O(M*N)
039	Combination Sum	Medium****	backtrack	-
040	Combination Sum II	Medium****	backtrack	-
046	Permutations	Medium****	backtrack DFS	-
047	Permutations II	Medium****	backtrack	-
049	Anagrams	Medium****	HashMap	-
056	Merge Intervals	Hard*****	-	O(N)
057	Insert Interval	Hard*****	-	O(N)
065	Valid Number	Hard*****	ifs	-
067	Add Binary	Easy****	-	O(N)
069	Sqrt(x)	Easy****	dichotomy	-
070	Climbing Stairs	Easy*****	Dynamic Programming	-
077	Combinations	Medium****	backtrack	-
078	Subsets	Medium****	backtrack	-
079	Word Search	Medium****	backtrack DFS	-
088	Merge Sorted Array	Easy*****	-	O(N)
090	Subsets II	Medium****	backtrack	-
091	Decode Ways	Medium****	DP	-
098	Validate Binary Search Tree	Medium*****	DFS	-
102	Binary Tree Level Order Traversal	Easy****	BFS Queue	-
125	Valid Palindrome	Easy*****	recursion	-
127	Word Ladder	Medium*****	Breadth-First Search	-
129	Sum Root to Leaf Numbers	Medium****	Depth-First Search	-
131	Palindrome Partitioning	Medium****	backtrack	-