Trapping Rain Water

1def trap(heights):
2    left = 0
3    right = len(heights) - 1
4    leftMax = 0
5    rightMax = 0
6    water = 0
7
8    while left < right:
9        if heights[left] < heights[right]:
10            if heights[left] >= leftMax:
11                leftMax = heights[left]
12            else:
13                water += leftMax - heights[left]
14            left += 1
15        else:
16            if heights[right] >= rightMax:
17                rightMax = heights[right]
18            else:
19                water += rightMax - heights[right]
20            right -= 1
21
22    return water