29. Trapping Rain Water
Given bar heights, compute how much rain water can be trapped.
Example 1:
Input: height = [0,1,0,2,1,0,1,3,2,1,2,1] Output: 6
Example 2:
Input: height = [4,2,0,3,2,5] Output: 9
Constraints:
0 <= height.length <= 2 * 10^4
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