FindAllDuplicates.java

package com.saikat;

import java.util.ArrayList;
import java.util.List;

public class FindAllDuplicates {
    public static void main(String[] args) {
        ArrayList<Integer> ans = new ArrayList<>();
        int[] nums = {4,3,2,7,8,2,3,1};
        System.out.println(findDisappearedNumbers(nums));
    }
    static List<Integer> findDisappearedNumbers(int[] nums) {
        int i = 0;
        while (i < nums.length){
            int correctIndex = nums[i] - 1;
            if (nums[i] != nums[correctIndex]){
                swap(nums,i,correctIndex);
            }else{
                i++;
            }
        }

        List<Integer> ans = new ArrayList<>();
        for(int index = 0; index < nums.length; index++){
            if(nums[index] != index + 1){
                ans.add(index + 1);
            }
        }
        return ans;
    }
    static void swap(int[] arr, int first, int second){
        int temp = arr[first];
        arr[first] = arr[second];
        arr[second] = temp;
    }
}