NeetCode 150

Arrays:
1. Contains-Duplicate - HashMap (Frequency Count) – O(n)
2. Valid Anagram - Array Frequency - O(n) & O(1)
3. Two Sum - HashMap One-pass - O(n)
4. Group Anagrams - Character Frequency O(n * k)
5. Top K frequent elements - HashMap + Bucket Sort - O(n)
6. Encode and Decode Strings - Length-Based Encoding - O(n)
7. Product of Array except self - Prefix * Suffix Product - O(n) & O(1) 
8. Valid Sudoku - 2D arrays row, col, and box - O(1)
9. Longest consecutive sequence - Hashset - O(n)