The basic idea is that we iterate through the input array and mark elements as negative using `nums[nums[i] -1] = -nums[nums[i]-1]`

. In this way all the numbers that we have seen will be marked as negative. In the second iteration, if a value is not marked as negative, it implies we have never seen that index before, so just add it to the return list.

```
public List<Integer> findDisappearedNumbers(int[] nums) {
List<Integer> ret = new ArrayList<Integer>();
for(int i = 0; i < nums.length; i++) {
int val = Math.abs(nums[i]) - 1;
if(nums[val] > 0) {
nums[val] = -nums[val];
}
}
for(int i = 0; i < nums.length; i++) {
if(nums[i] > 0) {
ret.add(i+1);
}
}
return ret;
}
```