Add Digits

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

For example:

Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.

Follow up:
Could you do it without any loop/recursion in O(1) runtime?
思路：可以按题意进行求解，代码如下：

class Solution {public:    int addDigits(int num) {        while(num > 9){        int tmp = 0;        while(num){            tmp += num % 10;            num = num / 10;        }        num = tmp;        }    return num;    }};

也可观察输出值，找规律，可以得出restult = （num - 1） % 9 + 1；
代码如下：

class Solution {public:    int addDigits(int num) {        return (num - 1) % 9 + 1;    }};