Longest Suffix

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.math.BigInteger;
import java.util.HashSet;
import java.util.StringTokenizer;

/**
 * <b> Algorithm on Codeforces. Problem E div 2 </b> </br>
 *
 * @author Huynh Quang Thao
 *
 */
public class Main {

    public static void main(String[] args) throws Exception {
        Main main = new Main();
        main.run();
    }

    public void run() throws Exception {
        InputReader sc = null;

        sc = new InputReader(System.in);
        sc = new InputReader(new FileInputStream(new File("input.txt")));

        int ntestcase = sc.nextInt();
        while (ntestcase-- > 0) {
            int n = sc.nextInt();
            HashSet<String> hash = new HashSet<String>();
            String res = "";
            for (int i = 0; i < n; i++) {
                String word = sc.next();
                for (int j = 0; j < Math.min(100, word.length()); j++) {
                    String prefix = word.substring(0, j+1);
                    // already has this suffix before. compare with res
                    if (hash.contains(prefix)) {
                        if (prefix.length() > res.length()) res = prefix;
                        else if (prefix.length() == res.length() && prefix.compareTo(res) < 0) res = prefix;
                    }
                    else hash.add(prefix);
                }
            }
            System.out.println(res);
        }
    }

    static class InputReader {
        public BufferedReader reader;

        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream));
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                }
                catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

        public float nextFloat() {
            return Float.parseFloat(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

        public BigInteger nextBigInteger() {
            return new BigInteger(next());
        }

    }
}