Data.List.find を使った巧妙な仕掛け

競プロ典型 90 問 16 日目 - Minimum Coins（★3）

import Control.Monad
import qualified Data.ByteString.Char8 as C
import Data.List
import Data.Maybe

main = sol <$> readLn <*> get >>= print

get = unfoldr (C.readInt . C.dropWhile (==' ')) <$> C.getLine

sol n as = fromJust $ find (\s -> any (f s) [z0,z0-1..0]) [z0..9999]
  where
  [a,b,c] = sort as
  z0 = n `div` c
  f s z = l>=0 && l `mod` (b-a)==0 && k>=0 && k `mod` (b-a)==0
    where
    m = n-c*z
    l = b*(s-z)-m
    k = m-a*(s-z)