Given n non-negative integers representing an elevation map, compute how much water it can trap after raining.
Example 1:
Input: height = [0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1]
Output: 6
Explanation: 6 units of water are trapped between the bars.
Example 2:
Input: height = [4, 2, 0, 3, 2, 5]
Output: 9