HackerRank – Missing Numbers

import java.util.*;

public class Solution{
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int[] a = new int[n];
        for(int i=0;i<n;i++){
            a[i] = in.nextInt();
        }
        int m = in.nextInt();
        int[] b = new int[m];
        for(int i=0;i<m;i++){
            b[i] = in.nextInt();
        }
        HashMap<Integer,Integer> h_a = new HashMap<Integer,Integer>();
        HashMap<Integer,Integer> h_b = new HashMap<Integer,Integer>();
        for(int i:a){
            if(h_a.containsKey(i)){
                h_a.put(i,h_a.get(i)+1);
            }
            else{
                h_a.put(i,1);
            }
        }
        for(int j:b){
            if(h_b.containsKey(j)){
                h_b.put(j,h_b.get(j)+1);
            }
            else{
                h_b.put(j,1);
            }
        }
        ArrayList<Integer> diff = new ArrayList<Integer>();
        for(Integer i:h_b.keySet()){
            if(h_a.get(i)==null || h_a.get(i)<h_b.get(i)){
                diff.add(i);
            }
        }
        Integer[] result = new Integer[diff.size()];
        diff.toArray(result);
        Arrays.sort(result);
        for(int i:result){
            System.out.print(i+" ");
        }
    }
}