42. Trapping Rain Water

Problem Link

Description

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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

Solution

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Python3

class Solution:
    def trap(self, height: List[int]) -> int:
        N = len(height)
        res = 0
 
        left, right = 0, N - 1
        leftMost, rightMost = height[0], height[-1]
 
        while left < right:
            leftMost = max(leftMost, height[left])
            rightMost = max(rightMost, height[right])
 
            if leftMost <= rightMost:
                res += leftMost - height[left]
                left += 1
            else:
                res += rightMost - height[right]
                right -= 1
 
        return res