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.

要求：Could you do it without any loop/recursion in O(1) runtime?
思考 2min

感觉这类题还是多看多积累经验得好。迭代实现是基本没问题的了，但 O(1) 复杂度，就是要一次计算出来。得推理出规律。。

推理：

整数 A 表示成 100a+10b+c，然后 =(100a+10b+c)%9=(a+99a+b+9b+c)%9=(a+b+c)%9

然后就自然得得到解。。return (num - 1) % 9 + 1;