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Trapping Rainwater

Problem Statement​

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.

Leetcode Link

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

Code​

Python Code

class Solution:
    def trap(self, height):
        waterLevel = []
        left = 0
        for h in height:
            left = max(left, h)
            waterLevel += [left] # over-fill it to left max height
        right = 0
        for i, h in reversed(list(enumerate(height))):
            right = max(right, h)
            waterLevel[i] = min(waterLevel[i], right) - h # drain to the right height
        return sum(waterLevel)