Calculate Factorial

Problem

Given a non-negative integer n, return the factorial of n.

The factorial of a number is the product of all positive integers up to that number. The factorial of 0 is defined as 1.

Examples

factorial(0)
// 1

factorial(1)
// 1

factorial(5)
// 120
/* The factorial of 5 is 5 * 4 * 3 * 2 * 1 = 120. */