42. Trapping Rain Water

HARD

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

class Solution:
    def trap(self, height: List[int]) -> int:
        leftMax = rightMax = 0
        totalWater = 0
        left = 0
        right = len(height) - 1
        while left <= right:
            if leftMax <= rightMax:
                if leftMax < height[left]:
                    leftMax = height[left]
                else:
                    totalWater += (leftMax - height[left])
                left += 1
            else:
                if rightMax < height[right]:
                    rightMax = height[right]
                else:
                    totalWater += (rightMax - height[right])
                right -= 1
        return totalWater