LeetCode 137 Single Number II 数组中除了一个数外，其他的数都出现了三次，找出这个只出现一次的数

Given an array of integers, every element appears three times except for one, which appears exactly once. Find that single one.
Note:Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

class Solution {
public:
    int singleNumber(vector<int>& nums) {
        int size=nums.size();
        if(size==0||nums.empty())
            return -1;
        int res=0;
        for(int i=0;i<32;++i)
        {
            int sum=0;
            for(int j=0;j<size;++j)
                sum+=(nums[j]>>i)&1;
            res+=(sum%3)<<i;
        }
        return res;
    }
};