Find duplicates in a vector

vector<int> findDuplicates(vector<int>& nums) {
        vector<int> result;
        for (int i = 0; i < nums.size(); ++i) {
            int index = abs(nums[i])-1;
            if (nums[index] < 0)
                result.push_back(index+1);
            nums[index] = -nums[index];
        }
        return result;
    }