[LeetCode 258] 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.
Follow up:Could you do it without any loop/recursion in O(1) runtime?

Method:
Digit root is the number that add all digits until the result has only one digit.

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