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) {
        int i=0;
        if(num==0) return 0;
        i=num%9;
        if(i==0) i=9;
        return i;
    }
}