find longest continuous sub number sequence.

longest.hs

import Data.List
import Data.Function

isSeq :: [Int] -> Bool
isSeq l =
  let l' = sort l
  in all (==(-1)) $ zipWith (-) l' (tail l')

longest :: [Int] -> [Int]
longest = maximumBy (compare `on` length)
          . map (head . filter isSeq . reverse . inits)
          . tails

main = do
    print $ longest [5,1,3,2]
    print $ longest [1,3,5,2,4]
    print $ longest [1,3,5,2]

{-
> ./Main
[1,3,2]
[1,3,5,2,4]
[2]
-}