12-Add Digits

寻找一个数的数根，用了暴力破解的方式，时间复杂度比较高
暂未想到O(1)的方式

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?

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