Frequency == 5

1. Two sum

2. atoi

3. 3Sum

4.Valid Parentheses

5. merge 2 sorted list

6. implement strStr()

7. Pow(x,n)

8. Merge Intervals

9. Insert Interval

10. valid number

11. climbing stairs

12. set matrix zeros

13.merge sorted array

14. Validate binary search tree

15. valid palindrome

16. Word Ladder