Interview Question for Software Engineer at PayPal:

There is a trick that need global variable or static variable to record the temper sum. so the function declaration cannot be same as loop version if don't use global or static variable.

the output parameter b will be the reverse result.

void rev(int d, int& b)
{
    b *= 10;
    if (d/10 == 0)
        b += d;
    else
    {
        b += d%10;
        rev(d/10, b);
    }
}

You can do it without the golbal or static variables:

int f(int num)
{
if((num/10) == 0)
  return num;
else
 return ( 10(num%10) + f(num/10) );
}