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.
思路：取各位数字相加，直至相加结果为一位数（<9），考虑边界情况
思路：规律是直接与9的取余
