[Leetcode] 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.

把一个非负整数的各个位相加，直到结果为个位数。

可以利用数根（digit root）: https://en.wikipedia.org/wiki/Digital_root

class Solution {
public:
    int addDigits(int num) {
        return 1+(num-1)%9;
    }
};