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?

枚举1-20可发现所有的数字之间都是有规律的，以9为周期，不断循环

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