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.length1 <= n <= 2 * 1040 <= height[i] <= 105