LeetCode 258----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?

输出	输入	输入	输入
0	0
1	1	10	19
2	2	11	20
3	3	12	21
4	4	13	22
5	5	14	23
6	6	15	24
7	7	16	25
8	8	17	26
9	9	18	27

从上表可以看出，除0外，输出值从1~9循环。

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