Divisible by 3?

Give a method if a number is divisible by 3 without using the modulo operator.

Answer:

Sum the digits recursively until we are left with 3 or 6 or 9

bool IsDivisible(int num)
{
    int sum_of_digits = 0;
    if ( num < 10 )
    {
        if ( num == 3 || num == 6 || num == 9 )
            return true;
        else
            return false;
    }
    while(num)
    {
        sum_of_digits += num % 10;
        num = num / 10;
    }
    return IsDivisible(sum_of_digits);
}