'''

def trap(self, height):

if len(height) < 2: return 0

l = 0

r = len(height) - 1

if height[l] < height[r]:

mid, d = 1, 1

else:

mid, d = r - 1, - 1

water = 0

while l < mid < r:

if d == 1:

if height[mid] < height[l]:

water += height[l] - height[mid]

mid += 1

elif height[mid] <= height[r]:

l = mid

mid += 1

else:

d = -1

l = mid

mid = r - 1

if d == -1:

if height[mid] < height[r]:

water += height[r] - height[mid]

mid -= 1

elif height[mid] <= height[l]:

r = mid

mid -= 1

else:

d = 1

r = mid

mid = l + 1

return water

'''