Given `n` non-negative integers representing an elevation map where the width of each bar is `1`, 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: The above elevation map (black section) is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped.

Example 2:

    Input: height = [4,2,0,3,2,5]
    Output: 9

Constraints:

- `n == height.length`
- `1 <= n <= 2 * 104`
- `0 <= height[i] <= 105`