# 42. Trapping Rain Water

**HARD**

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:**

![](https://assets.leetcode.com/uploads/2018/10/22/rainwatertrap.png)

```
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`

```python
class Solution:
    def trap(self, height: List[int]) -> int:
        leftMax = rightMax = 0
        totalWater = 0
        left = 0
        right = len(height) - 1
        while left <= right:
            if leftMax <= rightMax:
                if leftMax < height[left]:
                    leftMax = height[left]
                else:
                    totalWater += (leftMax - height[left])
                left += 1
            else:
                if rightMax < height[right]:
                    rightMax = height[right]
                else:
                    totalWater += (rightMax - height[right])
                right -= 1
        return totalWater
```