http://qandwhat.apps.runkite.com/i-failed-a-twitter-interview/

raining

module Raining where

waterVolume :: [Int] -> Int
waterVolume xs = water 0 0 [] xs
    where water maxHeight accWater puddles [] = accWater
          water maxHeight accWater puddles (x:xs)
            | x >= maxHeight   = water x (accWater + sum (map (flip subtract maxHeight) puddles)) [] xs
            | null puddles     = water maxHeight accWater [x] xs
            | x > head puddles = let (waterFilled, rest) = break (>= x) puddles in water maxHeight (accWater + sum (map (flip subtract x) waterFilled)) ((waterFilled >> return x) ++ rest) xs
            | otherwise        = water maxHeight accWater (x:puddles) xs

main :: IO ()
main = print . waterVolume $ [2,1,3,2,0,0,7,4,5,6,8,4,3,2,1,5]