/*
 * T12n01.java:  Two methods that compute n! -- one iterative, the other
 * recursive.  Note that the methods use the 'long' type instead of the
 * 'int' type.  This allows for larger integers to be stored.  Even so,
 * the factorial function grows so fast that we can only compute 20!.
 * If you need to compute even larger factorials, you'll want to use 
 * Java's BigInteger class.
 */

import java.util.*;

public class T12n01
{

        /*
         * factorialIterative(n) -- Computes n! iteratively and returns it.
         *
         *   Precondition:  n is a non-negative integer <= 20
         *   Postcondition: n! is returned
         */

    public static long factorialIterative(long n)
    {
        long product = 1;      // running product of integer sequence

        if (n == 0)            // Special case of 0! = 1  (but is this needed?)
            return 1;
        else {                 // General case of n! = 2 * ... * (n-1) * n
            for (int i = 2; i <= n; i++) {
                product *= i;
            }
            return product;
        }
    }


        /*
         * factorialRecursive(n) -- Computes n! recursively and returns it.
         *
         *   Precondition:  n is a non-negative integer <= 20
         *   Postcondition: n! is returned
         */

    public static long factorialRecursive(long n)
    {
        if (n == 0)            // Base case of 0! = 1  (again, is this needed?)
            return 1;
        else                   // General case of n! = n * (n-1)!
            return n * factorialRecursive(n-1);
    }


    public static void main (String [] args)
    {
        if (args.length < 1) {
            System.out.println("\n\tUSAGE:  java T12n01 <n>   and |n|! will "
                             + "be displayed if |n| <= 20.\n");
            System.exit(1);
        }

        long n = Math.abs(Long.parseLong(args[0]));

        if (n <= 20) {

            System.out.println("Iteratively, " + n + "! = " 
                             + factorialIterative(n));

            System.out.println("Recursively, " + n + "! = " 
                             + factorialRecursive(n));

        } else {

            System.out.println("A Java long cannot hold 21! or larger.");

        }
    }
}