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?

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