Added 19 new problem solutions in Java.

[Pikeras72]

These are the problems with the new Java solutions:

4. Median of Two Sorted Arrays
5. Integer to Roman
6. Roman to Integer
7. Longest Common Prefix
8. Valid Parentheses
9. Length of Last Word
10. Recover Binary Search Tree
11. Excel Sheet Column Number
12. Happy Number
13. Power of Two
14. Power of Three
15. Power of Four
16. Is Subsequence
17. Third Maximum Number
18. Distribute Candies to People
19. Minimum Remove to Make Valid Parentheses
20. Subtract the Product and Sum of Digits of an Integer
21. Can Make Arithmetic Progression From Sequence
22. Sign of the Product of an Array

Execution time of all solutions exceeds 50% to 99% of java solutions in leetcode.

My leetcode profile (https://leetcode.com/Pikeras/)