Problem:
Score : 100 points

Problem Statement

In AtCoder City, there are three stations numbered 1, 2, and 3.

Each of these stations is operated by one of the two railway companies, A and B. A string S of length 3 represents which company operates each station. If S_i is A, Company A operates Station i; if S_i is B, Company B operates Station i.

To improve the transportation condition, for each pair of a station operated by Company A and one operated by Company B, there will be a bus service connecting them.

Determine if there is a pair of stations that will be connected by a bus service.

Constraints

Each character of S is A or B.

|S| = 3

Input

Input is given from Standard Input in the following format:

S

Output

If there is a pair of stations that will be connected by a bus service, print Yes; otherwise, print No.

Sample Input 1

ABA

Sample Output 1

Yes

Company A operates Station 1 and 3, while Company B operates Station 2.

There will be a bus service between Station 1 and 2, and between Station 2 and 3, so print Yes.

Sample Input 2

BBA

Sample Output 2

Yes

Company B operates Station 1 and 2, while Company A operates Station 3.

There will be a bus service between Station 1 and 3, and between Station 2 and 3, so print Yes.

Sample Input 3

BBB

Sample Output 3

No

Company B operates all the stations. Thus, there will be no bus service, so print No.

Solution in Haskell (GHC 7.10.3) (judge verdict: Runtime Error)

import           Control.Monad
import           Data.Foldable
import qualified Data.Sequence as S
import           Data.Sequence ( Seq(..), (<|), (|>) )

data Direction = L | R

data Query = Reverse | InsertHead !Char | AppentLast !Char

main :: IO ()
main = do
  s <- S.fromList <$> getLine
  q <- readLn
  qs <- replicateM q readQuery
  putStrLn $ toList $ solve s L qs

solve :: Seq Char -> Direction -> [Query] -> Seq Char
solve s L []                  = s
solve s R []                  = S.reverse s
solve s L (Reverse      : qs) = solve s R qs
solve s R (Reverse      : qs) = solve s L qs
solve s L (InsertHead c : qs) = solve (c <| s) L qs
solve s R (InsertHead c : qs) = solve (s |> c) R qs
solve s L (AppentLast c : qs) = solve (s |> c) L qs
solve s R (AppentLast c : qs) = solve (c <| s) R qs

readQuery :: IO Query
readQuery = do
  q <- getLine
  case map head (words q) of
    ['1']         -> return $ Reverse
    ['2', '1', c] -> return $ InsertHead c
    ['2', '2', c] -> return $ AppentLast c