Arrays & Hashing problems

Using arrays and hash tables to track, count, and look up values fast.

1. Contains Duplicate (Easy)
2. Valid Anagram (Easy)
3. Two Sum (Easy)
4. Group Anagrams (Medium)
5. Top K Frequent Elements (Medium)
6. Product of Array Except Self (Medium)
7. Valid Sudoku (Medium)
8. Longest Consecutive Sequence (Medium)
9. Majority Element (Easy)
10. Subarray Sum Equals K (Medium)