import java.util.Hashtable;
import java.util.Map.Entry;
public class repeated_num {

    public static void main(String[] args) {
        int []arr={1,2,1,4,5,6,4,2,1};
        Hashtable<Integer,Boolean > ht = new Hashtable<Integer,Boolean >();

        for(int i=0;i<arr.length;i++)
            if(ht.get(arr[i]) == null)
                ht.put(arr[i],false);
            else
                ht.put(arr[i], true);



        for (Entry<Integer, Boolean> entry : ht.entrySet()) {
            Integer key = entry.getKey();
            Boolean value = entry.getValue();
            if(value)
                System.out.println(key);


        }


    }

}