Trapping Rain Water

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:
class Solution:
    def trap(self, height: List[int]) -> int:
        blocks = 0
        n = 0
        h = float('-inf')
        for hh in height:
            blocks += hh
            n += 1
            h = max(h, hh)
        total = 0
        left = 0
        right = n - 1
        for i in range(h):
            while height[left] <= i:
                left += 1
            while height[right] <= i:
                right -= 1
            total += right - left
        total -= (blocks - h)
        return total

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