258. Add Digits

问题：

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?

答案：

（提示：把数字按顺序写出来找规律）

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