Find 2 missing numbers in an array of integers with two missing values

Question

How do you do this? The values are unsorted but are of [1..n] Example array [3,1,2,5,7,8]. Answer: 4, 6

I saw this solution in

Answer 1

works for any number of missing elements: you can format the code a little .. but it works on duplicate and non duplicate entries also:

public static void main(String args[] ) throws Exception {

        Scanner input = new Scanner(System.in);
        System.out.println("Enter no. of students in the class");
        int N = input.nextInt();
        List l = new ArrayList();
        int Nn=N;
        System.out.println("Enter the roll numbers");
        for(int i=0;i