Compute how much water are trapped after raining (trapping rain water)

Problem:

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.

Input and output examples

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(object):
    def trap(self, height):
        """
        :type height: List[int]
        :rtype: int
        """
        if not height:
            return 0
        left = 0
        right = len(height) - 1
        left_max = height[left]
        right_max = height[right]
        res = 0
        while left < right:
            if height[left] < height[right]:
                if height[left] >= left_max:
                    left_max = height[left]
                else:
                    res += left_max - height[left]
                left += 1
            else:
                if height[right] >= right_max:
                    right_max = height[right]
                else:
                    res += right_max - height[right]
                right -= 1
        return res

Test the the solution:

s = Solution()
print(s.trap([0,1,0,2,1,0,1,3,2,1,2,1]))
print(s.trap([4,2,0,3,2,5]))
	

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