Problem:
Score : 200 points

Problem Statement

Iroha has a sequence of N strings S_1, S_2, ..., S_N. The length of each string is L.

She will concatenate all of the strings in some order, to produce a long string.

Among all strings that she can produce in this way, find the lexicographically smallest one.

Here, a string s=s_1s_2s_3...s_n is lexicographically smaller than another string t=t_1t_2t_3...t_m if and only if one of the following holds:

There exists an index i(1≦i≦min(n,m)), such that s_j = t_j for all indices j(1≦j<i), and s_i<t_i.

s_i = t_i for all integers i(1≦i≦min(n,m)), and n<m.

Constraints

1 ≦ N, L ≦ 100

For each i, the length of S_i equals L.

For each i, S_i consists of lowercase letters.

Input

The input is given from Standard Input in the following format:

N L
S_1
S_2
:
S_N

Output

Print the lexicographically smallest string that Iroha can produce.

Sample Input 1

3 3
dxx
axx
cxx

Sample Output 1

axxcxxdxx

The following order should be used: axx, cxx, dxx.

Solution in Haskell (GHC 7.10.3) (judge verdict: Accepted)

module Main where

import           Control.Monad
import           Debug.Trace
import qualified Data.Vector.Unboxed           as VU
import           Data.List
import qualified Data.ByteString               as BS
import qualified Data.ByteString.Char8         as BSC8
import           Data.Char
import           Data.Maybe

tuplify2 (x : y : _) = (x, y)
tuplify2 _           = undefined

getVUIntFromInputLineAtBSC8 :: IO (VU.Vector Int)
getVUIntFromInputLineAtBSC8 =
    VU.unfoldr (BSC8.readInt . BSC8.dropWhile isSpace) <$> BSC8.getLine
getVUIntListFromInputLineAtBSC8 =
    VU.unfoldr (BSC8.readInt . BSC8.dropWhile isSpace) <$> BSC8.getLine

readInt = fst . fromJust . BSC8.readInt
readIntTuple = tuplify2 . map readInt . BSC8.words
readIntList = map readInt . BSC8.words

getInt = readInt <$> BSC8.getLine
getIntList = readIntList <$> BSC8.getLine
getIntNList n = map readIntList <$> replicateM (fromIntegral n) BSC8.getLine
getIntMatrix = map readIntList . BSC8.lines <$> BS.getContents
getIntTuple = readIntTuple <$> BSC8.getLine
getIntNTuples n = map readIntTuple <$> replicateM (fromIntegral n) BSC8.getLine
getIntTuples = map readIntTuple . BSC8.lines <$> BSC8.getContents

main :: IO ()
main = do
    [aN,aL] <- getIntList
    aS <- foldl (\acc x -> do
        aS <- getLine
        accString <- acc
        return (accString ++ [aS])) (return []) [1..aN]
    let ret = foldl (\acc x->acc ++ x) [] (sort aS)
    putStrLn ret