問題描述：

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?

陣列中的只有一個元素出現了一次，其他的元素均出現了三次，對於這些整數的每一位來說，它的位值也會相應的出現3次，即對3取餘問為0.

過程詳見程式碼：

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