leetcode: 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 is able to trap after raining.
# 
# For example, 
# Given [0,1,0,2,1,0,1,3,2,1,2,1], return 6.

# 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.

class Solution():
    def trap(self, x):
        res = 0
        if x:
            left, mid, right = 0, x.index(max(x)), len(x)-1
            for i in range(mid):
                if x[left] < x[i]:
                    left = i
                res += x[left] - x[i]
            for i in reversed(range(mid+1, len(x))):
                if x[right] < x[i]:
                    right = i
                res += x[right] - x[i]
        return res


if __name__ == "__main__":
    assert Solution().trap([0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1]) == 6