神一样的代码：

public static int singleNumber(int[] nums) {
    int once = 0;
    int twice = 0;
    for (int i = 0; i < nums.length; i++) {
        twice |= once & nums[i];
        once ^= nums[i];
        int not_three = ~(once & twice);
        once = not_three & once;
        twice = not_three & twice;
    }
    return once;
}