import java.util.*;
public class Main{
    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        while(n-->0){
            int m = scanner.nextInt();
            String[] set = new String[m];
            for(int i = 0; i < m ; i++)
                set[i] = scanner.next();
            Arrays.sort(set , (String o1, String o2) -> {
                String s1 = o1 + o2;
                String s2 = o2 + o1;
                if (s1.compareTo(s2) < 0)
                    return -1;
                else
                    return 1;
            });
            StringBuffer sb = new StringBuffer() ;
            for(String ss : set )
                sb.append(ss);
            System.out.println(sb.toString());
        }
    }
}