**Idea is hear pretty neat. You just have to concentrate that size of array is n and numbers are also between 1->n so just look for missing indexed.**

```
vector<int> findDisappearedNumbers(vector<int>& nums) {
for(int i=0; i<nums.size(); i++){
int val = abs(nums[i]) - 1;
if(nums[val] > 0)
nums[val] = - nums[val]; // Negative to tell that visited that index
}
vector<int> returnV; // retur vector for storing missing elements
for(int i=0; i<nums.size(); i++){
if(nums[i] > 0){
returnV.push_back(i+1); // Postive index shows that this number never appeared in array
}
}
return returnV;
}
```