Table of Contents

## 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:6Explanation: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 * 10`

^{4}`0 <= height[i] <= 10`

^{5}

## 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)