【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?

Hint:

A naive implementation of the above process is trivial. Could you come up with other methods?

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