UVA - 11362 - Phone List

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;

public class Main {
   
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringBuffer sb = new StringBuffer("");
        int cases=Integer.parseInt(br.readLine().trim());
        for(int z=0;z<cases;z++) {
            int n=Integer.parseInt(br.readLine().trim());
            String[]str=new String[n];
            for(int j=0;j<n;j++){
                str[j]=br.readLine().trim();
            }
            Arrays.sort(str);
            boolean flag=false;
            for(int i=0;i<n-1;i++){
                   boolean oops=true;
                   for(int l=0;l<Math.min(str[i].length(), str[i+1].length());l++){
                       if(str[i].charAt(l)!=str[i+1].charAt(l)){
                           oops=false;
                           break;
                       }
                }
                if(oops){
                       flag=true;
                       break;
                   }
            }
            if(flag){
                sb.append("NO\n");
            }else{
                sb.append("YES\n");
            }
        }
        System.out.print(sb);
    }
}