[leetcode-258]Add Digits（C）

问题描述：
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?

分析：这道题就是找规律。

代码如下：4ms

int addDigits(int num) {    return (num-1)%9+1;   }