Fastest method for adding/summing the individual digit components of a number

Question

I saw a question on a math forum a while back where a person was discussing adding up the digits in a number over and over again until a single digit is achieved. (i.e. \"362\"

Answer 1

int digit_sum(int n)
Do
    if (n<10) return n;
    Exit do
    else

    n=n%10 + digit_sum(n/10);

Loop