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.

实现：

class Solution {
public:
    int addDigits(int num) {
        int result = num;
        while (true) {
            if (result < 10) {
                return result;
            }
            num = result;
            result = 0;
            while (num) {
                result = result + num % 10;
                num = num / 10;
            }
        }
    }
};