1. Two Sum Medium
2. Valid Parentheses Easy
3. Merge Two Sorted Lists Easy
4. Intersection of Two Linked Lists Easy
5. Valid Anagram Easy
6. Two Sum II - Input array is sorted Medium
7. Rotate Image Medium
8. Reverse Words in a String II Medium
9. Gray Code Medium
10. Reverse Linked List - Iterative Easy 206(2). Reverse Second Half of Linked Lists Medium
11. Group Anagrams- Sort, Hashmap Medium
12. Merge K Sorted Lists - Priority Queue Hard
13. Minimal Path Sum Medium
14. Binary Tree Right Side View(DFS and BFS) Medium
15. Number of Islands - BFS, Union and Find Medium
16. Subsets - Bit Manipulation Medium K Nearest Point to the Origin -- heap
17. Valid Binary Search Tree Medium

BST:对于当前node，左子树最大节点值 < node.val < 右子树最小节点值 235. Lowest Common Ancestor of a Binary Search Tree - BST Property Easy 102. Binary Tree Level Order Traversal - BFS Easy 100. Same Tree

8. String to Integer(atoi) Easy
9. Min Stack Easy
10. Count Primes Easy

----------------------- To be finished ------------------------ largest-bst-subtree 239. Sliding Maximum Window - deque Hard 146. LRU Cache Hard 5. Longest Palindromic Substring - Center Expand/DP Medium