258 Add Digits

Given a non-negative integernum, repeatedly add all its digits until the result has only one digit.

Example:

Input:
38
Output:
 2 

Explanation: 
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?

     1    1
     2    2
     3    3
     4    4
     5    5
     6    6
     7    7
     8    8
     9    9


     10    1
     11    2
     12    3
     13    4
     14    5
     15    6
     16    7
     17    8
     18    9
     19    1
     20    2
     time : O(1);
     space : O(1):

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