42. 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 * 10^4
• 0 <= height[i] <= 10^5

Solution

class Solution:
    def trap(self, height: list[int]) -> int:
        left, right = 0, len(height) - 1
        leftMax, rightMax = 0, 0
        total = 0

        while left < right:
            leftMax = max(leftMax, height[left])
            rightMax = max(rightMax, height[right])

            if height[left] < height[right]:
                total += leftMax - height[left]
                left += 1
            else:
                total += rightMax - height[right]
                right -= 1

        return total