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 Scala (2.11.7) (judge verdict: Accepted)

import scala.io.StdIn._

object Helpers {
  def readIntLine = readLine.split(" ").map(_.toInt)
  def readInts(n: Int) = Array.fill(n)(readInt)
  def readLines(n: Int) = Array.fill(n)(readLine)

  def splitDigits(i: Int): Seq[Int] = i.toString.map(_.asDigit)

  def log[A](x: A): A = {println(x); x}

  implicit class IntImplicits(val i: Int) {
    def isEven = i % 2 == 0
    def isOdd = !isEven

    def ** (exp: Int): Int = scala.math.pow(i, exp).intValue
  }

  implicit class LongImplicits(val i: Long) {
    def isEven = i % 2 == 0
    def isOdd = !isEven

    def ** (exp: Long): Long = scala.math.pow(i, exp).longValue
  }

  def mycomb[T](n: Int, l: List[T]): List[List[T]] =
    n match {
      case 0 => List(List())
      case _ => for(el <- l;
                sl <- mycomb(n-1, l dropWhile { _ != el } ))
                yield el :: sl
  }
  
  def comb[T](n: Int, l: List[T]): List[List[T]] = mycomb(n, l.distinct)
}

object Main extends App {
  import Helpers._

  val Array(n, l) = readIntLine
  val ss = readLines(n)

  println(ss.sorted.reduce(_ + _))
}