frc wars

frc.hs

import Control.Monad.Trans.State
import Data.List

frc sequ = fst $ execState (mapM_ check sequ) (Nothing,[])

check :: Eq a => a -> State (Maybe a, [a]) ()
check x = do
    (status,prev) <- get
    case status of
        Just x -> 
            put (status,prev)
        Nothing ->
            if x `elem` prev then
                put (Just x, prev)
            else
                put (status, x:prev)

frc2, frc3 :: Eq a => [a] -> Maybe a
frc2 = safeHead . fst . foldr (\x (s,l) -> 
        if x `elem` l then
            (x:s,l)
        else (s,x:l)
    ) ([],[])
    where
        safeHead [] = Nothing
        safeHead (x:_) = Just x

frc3 = f []
    where 
        f _ [] = Nothing
        f l (x:xs) = if x `elem` l then Just x else f (x:l) xs
    

main = print . frc2 . flip take testData2 . read =<< getLine
    where
        testData2 =  cycle "ab"