第一轮
The number in the array represents the height of columns.
Find the maximum amount of water that can be filled in the entire structure.
Example:
Input: [1, 2, 1, 3, 1, 2, 1, 3, 1, 2, 1],
Output: 7
第二轮
Remove invalid parenthesis from a String. https://leetcode.com/problems/remove-invalid-parentheses/