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==||nums.empty())
             return -;
         int res=;
         for(int i=;i<;++i)
         {
             int sum=;
             for(int j=;j<size;++j)
                 sum+=(nums[j]>>i)&;
             res+=(sum%)<<i;
         }
         return res;
     }
 };