class Solution {
public:
    int findDuplicate(vector<int>& nums) {
        for(auto i = 0; i < nums.size(); ++i)
        {
            auto mark = abs(nums[i]) - 1;
            if (nums[mark] > 0 ) nums[mark] *= -1;
            else 
                return abs(nums[i]);
        }


        return -1;
        
    }
};