Add Digits

题目：

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.

Follow up:
Could you do it without any loop/recursion in O(1) runtime?

解析：

就是把一个整数的每个数字分离相加

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