Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.

For example,
Given[0,1,0,2,1,0,1,3,2,1,2,1], return6.

The above elevation map 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.Thanks Marcos for contributing this image!

URL: https://leetcode.com/problems/trapping-rain-water/

class Solution(object):
    def trap(self, height):
        """
        :type height: List[int]
        :rtype: int
        """
        maxseenright = 0
        maxseenright_arr = [0]*len(height)
        maxseenleft = 0
        rainwater = 0

        for i in range(len(height) - 1, -1, -1):
            if height[i] > maxseenright:
                maxseenright_arr[i] = height[i]
                maxseenright = height[i]
            else:
                maxseenright_arr[i] = maxseenright


        for i in range(0, len(height)):
            rainwater = rainwater + max(min(maxseenright_arr[i], maxseenleft) - height[i],0)
            if height[i] > maxseenleft:
                maxseenleft = height[i]

        return rainwater