class Solution {
    public List<Integer> findDisappearedNumbers(int[] nums) {
       for(int i=0; i< nums.length;){
           int element = nums[i]-1;
           if(nums[i]!=nums[element]){
               // swap
               int temp = nums[i];
               nums[i]=nums[element];
               nums[element]=temp;
           }else{
               i++;
           }
           for(int l=0; l<nums.length; l++){
               System.out.print(nums[l]);
               System.out.print(" ");
           }
           System.out.println("");
       }
       List<Integer> ans = new ArrayList<Integer>();
       for(int j=0; j < nums.length; j++){
           System.out.println(nums[j]);
           if(nums[j]!=j+1){
               ans.add(j+1);
           }
       }
       return ans;
    }
}

Result

Input: [4,3,2,7,8,2,3,1]

7 3 2 4 8 2 3 1 
3 3 2 4 8 2 7 1 
2 3 3 4 8 2 7 1 
3 2 3 4 8 2 7 1 
3 2 3 4 8 2 7 1 
3 2 3 4 8 2 7 1 
3 2 3 4 8 2 7 1 
3 2 3 4 8 2 7 1 
3 2 3 4 1 2 7 8 
1 2 3 4 3 2 7 8 
1 2 3 4 3 2 7 8 
1 2 3 4 3 2 7 8 
1 2 3 4 3 2 7 8 
1 2 3 4 3 2 7 8 
1
2
3
4
3
2
7
8

Output:
[5,6]