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)