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