448. Find All Numbers Disappeared in an Array

class Solution {
public:
    vector<int> findDisappearedNumbers(vector<int>& nums) 
    {
        vector<int> ans;
        int idx = 0;

       for(int i=0; i<nums.size(); i++)
       {
           if(nums[i]>0)
           {
               idx=nums[i]-1;
           }
           else
           {
                idx = nums[i]*-1-1;
           }

           if(nums[idx]>0)
           {
               nums[idx] = -nums[idx];
           }
       } 

       for(int i=0; i<nums.size(); i++)
       {
           if(nums[i]>0)
           {
               ans.push_back(i+1);
           }
       }
       return ans;
    }
};