[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)