using System;
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine("3 * 7 = {0}", multiply(3, 7));
Console.WriteLine("3 * 0 = {0}", multiply(3, 0));
Console.WriteLine("0 * 3 = {0}", multiply(0, 3));
Console.WriteLine("3 * -5 = {0}", multiply(3, -5));
Console.WriteLine("-3 * 6 = {0}", multiply(-3, 6));
}
static int multiply(int x, int y)
{
if (y > 0)
return (x + multiply(x, y - 1));
if (y < 0)
return -multiply(x, -y);
return 0;
}
}
}
/*
run:
3 * 7 = 21
3 * 0 = 0
0 * 3 = 0
3 * -5 = -15
-3 * 6 = -18
*/