Add Digits

Question:

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: integer root

Solution:

java:

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