leetcode Trapping Rain Water

class Solution(object):
def trap(self, height):
"""
:type height: List[int]
:rtype: int
"""
r = 0
begin =0
end = len(height)-1
while (begin<end):
left,right = begin+1,end-1
hl = height[begin]
hr = height[end]
if hl<=hr:
while(height[left]<hl):
r+=hl-height[left]
left+=1
while left<len(height)-1 and height[left+1]>=height[left]:
left+=1
begin = left
left+=1
else:
while height[right]<hr:
r+=hr-height[right]
right-=1
while right>0 and height[right-1]>=height[right]:
right-=1
end = right
right-=1
return r