Given a positive number `n`, the factorial of that number, denoted
`n!`, is equal to
`1 × 2 × ... × (n-1) × n`.
It can also be computed recursively: `n! = n × (n-1)!`,
for all `n` greater than or equal to zero.

For example,

3! = 3 × 2! = 3 × 2 × 1! = 3 × 2 × 1 = 3 × 2 = 6

By convention, 0! is 1.