The 136 single number array has two occurrences of the number except one, to find only one occurrence

Given an array of integers, the remaining elements appear two times, except for an element. Please find out this element that appears only once.
Note:
Your algorithm should be a linear time-of-day complexity. Can you implement it without extra space?

See: https://leetcode.com/problems/single-number/description/

Class Solution {public:    int singlenumber (vector<int>& nums) {        int n=nums.size ();        if (n==0| | Nums.empty ())        {            return-1;        }        int num=0;        for (int n:nums)        {            num^=n;        }        return num;    }};

The 136 single number array has two occurrences of the number except one, to find only one occurrence