FindAllNumbers.java

package com.devi;

import java.util.*;

public class FindAllNumbers {
    public static void main(String[] args) {
        int[] arr = {4,3,2,7,8,2,3,1};
        System.out.println(findAllNumbers(arr));
    }

    public static List<Integer> findAllNumbers(int[] arr) {

        int i = 0;
        while (i < arr.length) {
            int correct = arr[i]-1;
            if (arr[i] != arr[correct]) {
                swap (arr, i , correct);
            } else {
                i++;
            }
        }
        //just find missing numbers
        List<Integer> ans = new ArrayList<>();

        for (int index = 0; index < arr.length; index++) {
            if (arr[index] != index+1) {
                ans.add(index + 1);
            }
        }
        return ans;
    }

    public static void swap(int[] arr, int first, int second) {
        int temp = arr[first];
        arr[first] = arr[second];
        arr[second] = temp;
    }
}