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?

1. What are all the possible results?
2. How do they occur, periodically or randomly?

   public int addDigits(int num) {  
     //1-9  
     //10 11 13 ..18   
     //19 20 21 ..27  
     //28 29 30 ..36  
     //37   
     return (num - 1) % 9 + 1;
   }