442. Find All Duplicates in an Array

class Solution {
public:
    vector<int> findDuplicates(vector<int>& nums) {
        int n = nums.size();
        int index = 0;
        vector<int> ans;

       for(int i=0; i<n; i++)
       {
           index = abs(nums[i]) - 1;

           if(nums[index]>0)
           {
               nums[index] = -1 * nums[index];
           }
           else
           {
               ans.push_back(index+1);
           }
       } 

       return ans;
    }
};