Skip to content

42. Trapping Rain Water

Difficulty Topics

Description

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

trapping-rain-water.py
class Solution:
    def trap(self, height: List[int]) -> int:
        n = len(height)
        if n < 3: return 0

        i, j = 0, n - 1
        leftMost, rightMost = height[i], height[j]
        res = 0

        while i <= j:
            leftMost = max(leftMost, height[i])
            rightMost = max(rightMost, height[j])

            if leftMost < rightMost:
                res += leftMost - height[i]
                i += 1
            else:
                res += rightMost - height[j]
                j -= 1

        return res