Trapping rain water

We will iterate over the whole list, and also check the left and right side for the highest value, then substruct the value itself from the highest value, ans add it to a running sum. Then we return the result after the iteration is over.

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

Time Complexity: O(n)
Space Complexity: O(1)