Problem4

problem4.hs

problem4 :: [Int] -> Int
problem4 = read . foldl1 (++) . map show . sortBy f

f:: Int -> Int -> Ordering
f x y | v1 == v2 = EQ
      | v1 < v2  = GT
      | v1 > v2  = LT
  where
    v1 = concatInt x y
    v2 = concatInt y x

toString :: Int -> Int -> String
toString a b = show a ++ show b

concatInt :: Int -> Int -> Int
concatInt a = read . toString a