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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)