A concise-ish solution to http://programmingpraxis.com/2013/11/15/twitter-puddle/

twitter_puddle.py

def twitter_puddle(wall):
    height = max(wall)
    total = 0
    for x in range(1, len(wall)-2):
        lowest_wall = min(max(wall[:x]), max(wall[x+1:]))
        if lowest_wall > wall[x]:
            total += lowest_wall - wall[x]
    return total

assert(twitter_puddle([2, 5, 1, 2, 3, 4, 7, 7, 6]) == 10)