# [Solved] 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. ## Question

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`

## Python Solution

```class Solution:
def trap(self, A: List[int]) -> int:
water = 0
left = 0
right = len(A)-1

left_biggest_wall = 0
right_biggest_wall = 0

while left < right:
if A[left] < A[right]:
left_biggest_wall = max(left_biggest_wall,A[left])
if A[left] < left_biggest_wall:
water += left_biggest_wall-A[left]
left +=1

else:
right_biggest_wall = max(right_biggest_wall,A[right])
if A[right] < right_biggest_wall:
water += right_biggest_wall-A[right]
right-=1

return(water)```