1. We had done a factorial related exercise in one of the previous sections but to recap, the factorial of a number is the product of all the numbers up till that number.
For-example the factorial of 5 is 1 x 2 x 3 x 4 x 5 which is 120, similarly the factorial of 4 is 24. There are two additional rules:
- Factorial of 0 is 1;
- Factorial of a negative number is undefined so it should output undefined;
2. For solving the task, following the following steps:
- Fill in the blank for the factorial method with appropriate parameter definition. It expects an integer. Figure out what should be the name of the parameter by reading the method's inner code;
- If the passed argument is 0, it should show a fixed output which is mentioned in the hints;
- The loop should continue up till (including the number) the passed integer. The result variable should be updated with the product of itself with the loop variable. In short: result *= i;
- If the input number is negative (invalid), it should output "undefined".

using System;

namespace ConsoleApp
{
    internal class Program
    {
        static void factorial(int n)
        {
            if(n == 0)
            {
                Console.WriteLine(1);
            }
            else if (n > 0)
            {
                int result = 1;
                for (int i = 2; i <= n; i++)
                {
                    result *= i;
                }
                Console.WriteLine(result);
            }
            else
            {
                Console.WriteLine("undefined");
            }

        }
        static void Main(string[] args)
        {
            factorial(-1);
            factorial(0);
            factorial(5);
        }
    }
}