leetcode学习篇五——Add Digits

题目如下：
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?
题目要求能不能在O(1)的时间里求出来，所以感觉应该是有某种规律可以直接推出结果的，所以从1开始列了数字然后算出结果，果真存在规律，从1到9然后又是从1到9，得到一下代码，通过。

class Solution {
public:
    int addDigits(int num) {
        return num==0?0:(num%9==0?9:num%9);
    }
};