First of all we must understand the meaning of factorial:

The factorial function multiply all numbers from our chosen number down to 1.

For instance:

• 3! = 3 x 2! = 3 x 2 x 1 = 6
• 5! = 5 x 4! = 5 x 4 x 3 x 2 x 1 = 120
• 1! = 1
• 0! = 1

Way of thinking:

• We need a recursive function to calculate multiply of ordered numbers.
• We must return 1 when we get 0 and we must stop recursive function.