Single Number II

Example 1:

Input: [2,2,3,2]
Output: 3
Example 2:

Input: [0,1,0,1,0,1,99]
Output: 99

class Solution
{
public:
    int singleNumber(vector<int>& nums)
    {
        if(nums.empty())
            return -1;
        else if(nums.size()==1)
            return nums.front();

        int res=0;
        for(int i=31;i>=0;--i)
        {
            int mask=1<<i;
            int count=0;
            for(int j=0;j<nums.size();++j)
                if(mask&nums.at(j))
                    ++count;
            if(count%3)
                res|=mask;
        }
        return res;
    }
};