'''
[easy,math]
Given a non-negative integer num, 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?

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/add-digits
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。
'''