Birthday party

java.code

import java.io.*;
import java.util.*;

public class Main{

public static void main(String[] args) throws Exception {
  Scanner scn = new Scanner(System.in);
      int n1 = scn.nextInt();
      int[] a1 = new int[n1];

      for (int i = 0; i < n1; i++) {
         a1[i] = scn.nextInt();
      }

 int n2 = scn.nextInt();
      int[] a2 = new int[n2];

      for (int i = 0; i < n2; i++) {
         a2[i] = scn.nextInt();
      }
    System.out.println(intersection(a1,a2));
}

public static int[] intersection(int[] nums1, int[] nums2) {
    HashSet<Integer> set1 = new HashSet<Integer>();
    for (Integer n : nums1) set1.add(n);
    HashSet<Integer> set2 = new HashSet<Integer>();
    for (Integer n : nums2) set2.add(n);

    set1.retainAll(set2);

    int [] output = new int[set1.size()];
    int idx = 0;
    for (int s : set1) output[idx++] = s;
    return output;
  }
}