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.
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.
Input: height = [4,2,0,3,2,5] Output: 9
n == height.length
1 <= n <= 2 * 104
0 <= height[i] <= 105