Find the Duplicate Number

class Solution {
public:
    int findDuplicate(vector<int>& nums) {
        int sum =0;
        int n = nums.size();
        int k;
        for(int i=0; i<n; i++){
            if(nums[i]<0) k = - nums[i];
            else k = nums[i];

            if(nums[k-1]<0){
                return k;
            }
            else{
                nums[k-1] = -nums[k-1];
            }
        }

        return 0;
    }
};