Find the Duplicate Number

class Solution {
public:
    int findDuplicate(vector<int>& nums) {
        int n = nums.size();

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

            if(nums[indx]>0)nums[indx] *=-1;
            else return indx+1;

        }

        return -1;
    }
};