Arrays and Hashing

1. Contains Duplicate
2. Valid Anagram
3. Two Sum
4. Group Anagrams
5. Top K Frequent Elements
6. Encode and Decode Strings
7. Products of Array Except Self
8. Valid Sodoku
9. Longest Consecutive Sequence

Two Pointers

1. Valid Palindrome
2. Two Integer Sum II
3. 3 Sum