- Create a new array of the same size as the input array
- Iterate through the original array and place the numbers in the new array at an index number that matches the value of the number. For ex: if you find no. 5, place it at the 5th index in the new array.
- The new array will now have some numbers and some zeros. All those zeros have the index number which is the missing number that we want.
- Iterate through the new array and add the index number to the list whenever you find a zero.
- Return the list.

Code is attached below:

'''

class Solution {

public List<Integer> findDisappearedNumbers(int[] nums) {

```
List<Integer> listy = new ArrayList<Integer>();
int[] numsFinal = new int[nums.length];
for(int i=0;i<nums.length;i++)
{
numsFinal[nums[i]-1]=nums[i];
}
for(int i=0;i<numsFinal.length;i++)
{
if(numsFinal[i]==0)
{
listy.add(i+1);
}
}
return listy;
}
```

}

'''