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     public int addDigits(int num) 
 2     {
 3         if (num/10==0) return num;
 4         int sum = 0;
 5         while(num!=0)
 6         {
 7             sum= sum+num%10;
 8             num= num/10;
 9         }
10         return addDigits(sum);
11         
12     }

Add Digits

原文：http://www.cnblogs.com/hygeia/p/4799858.html