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.
找规律
class Solution { public: int addDigits(int num) { return 1 + (num - 1) % 9; } };
原文：http://www.cnblogs.com/pk28/p/7295974.html