258. Add Digits

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

For example:

Givennum = 38, the process is like:3 + 8 = 11,1 + 1 = 2. Since2has only one digit, return it.

Follow up:

Could you do it without any loop/recursion in O(1) runtime?

 /// 这个follow up 有点贱， 不是编程逻辑问题 纯数学问题， return 1+ （n-1）%9 ;  就酱！！！ 别问我为什么， 如果面试出这个问题， 这人有点贱， 呵呵呵。

正常解法很简单