water.py

def count_water(walls):
    n = len(walls)
    stack = list() # use a list as a stack
    stack.append((walls[0], 0)) # base case
    
    # go over the list of walls once
    for i in range(1, n):
        current_wall_height = walls[i]
        prev_wall_height = stack[-1][0]

        if current_wall_height < prev_wall_height:
            # draw new box
            stack.append((current_wall_height, 1)) # push
        else:
            # this wall is higher equal to the prev
            # see if we can merge with prev boxes
            new_width = 1
            prev_height = 0
            while len(stack) > 0:
                prev_height = stack[-1][0]
                if prev_height > current_wall_height:
                    break
                else:
                    (_, width) = stack.pop()
                    new_width += width

            # merged box
            stack.append((min(prev_height, current_wall_height), new_width))

    # [(11, 1), (8, 3), (6, 1)]
    print(stack)

    water = 0
    for (height, width) in stack:
        water += height * width * 1000

    return water
  
walls = [11, 13, 7, 6, 8, 6]
water_count = count_water(walls)
print(water_count)