Added 19 new problem solutions in Java.

[Pikeras72]

These are the problems with new Java solutions and explanations:

4 - Median of Two Sorted Arrays
12 - Integer to Roman
13 - Roman to Integer
14 - Longest Common Prefix
20 - Valid Parentheses
58 - Length of Last Word
99 - Recover Binary Search Tree
171 - Excel Sheet Column Number
202 - Happy Number
231 - Power of Two
326 - Power of Three
342 - Power of Four
392 - Is Subsequence
414 - Third Maximum Number
1103 - Distribute Candies to People
1249 - Minimum Remove to Make Valid Parentheses
1281 - Subtract the Product and Sum of Digits of an Integer
1502 - Can Make Arithmetic Progression From Sequence
1822 - 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/)