Factorial

A factorial is the product (multiplication result) of all positive integers less than or equal to a given number. For example:

• 1! = 1 = 1
• 2! = 2 * 1 = 2
• 3! = 3 * 2 * 1 = 6
• 4! = 4 * 3 * 2 * 1 = 24
• 5! = 5 * 4 * 3 * 2 * 1 = 120

There's also a special case for zero:

• 0! = 1

It's just 1 by definition of the fancy math folks (ivory tower types).

Assignment

Complete the factorial() function. It should return the factorial of a given number.

In mathematics, the ! symbol denotes a factorial, but is not a valid Python Factorial operator. Use a loop and multiplication to compute the proper result.