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.

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

class Solution {
private:
    int result=0;
public:
    int addDigits(int num) {
        if(num/10==0) return num;
        int b=num%10;
        int a=num/10;
        result=addDigits(a)+b;
        return addDigits(result);
        
    }
};