Find missing numbers in array through n

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

class MyCode {


/**
    * 4 Missing Number
    *
    * Given range of 1 to N and an array of unique integers that are within that
    * range, return the missing integers not found in the array
    *
    * Parameters
    * Input: n {Integer}
    * Input: arr {Array of Integers}
    * Output: {ArrayList of Integers}
    *
    * Constraints
    * Time: O(N)
    * Space: O(N)
    *
    * Examples
    * `4, [1, 4, 2] --> [3]`
    * `8, [4, 7, 1, 6] --> [2, 3, 5, 8]`
    * `6, [6, 4, 2, 1] --> [3, 5]`
    */
   public static void main (String[] args) {

    int[] arr = new int[]{1,4,2,3};
    ArrayList<Integer> list = new ArrayList<Integer>();
    list = missingNumber(9, arr);
    System.out.println(list);
    }


    public static ArrayList<Integer> missingNumber(int n, int[] arr) {
      int max = n; //for readability
      Arrays.sort(arr);
      //get min from the sorted input array
      int min = arr[0];
      int len = n - min +1;
      ArrayList<Integer> list = new ArrayList<Integer>();
      HashSet<Integer> set = new HashSet<Integer>();
      for(int i = 0; i < arr.length; i++){
       set.add(arr[i]);
      }
      for(int index = min; index < max; index++)
        if(!set.contains(index)){
          list.add(index);
      }
      if(!set.contains(max)){
        list.add(max);
      }
      System.out.println(list);
      return list;
    }
}