Write a C++ programming to add repeatedly all digits of a given non-negative number until the result has only one digit

#include <iostream>
using namespace std;

int addDigits(int num) {
        return num - (num - 1) / 9 * 9;
    }

int main(void)
{
    int n = 15;
    cout << "\nInitial number is "<< n << " single digit number is " << addDigits(n) << endl; 
    n = 57;
    cout << "\nInitial number is "<< n << " single digit number is " << addDigits(n) << endl; 
    return 0;   
}