This code is kind of incomplete

Question

It is necessary define return 1 for input 0, and error message if the input is negative. I did it in https://onlinegdb.com/SyP_Mw9PI
If you want to learn how to do it.

Answer 1

I don't get it..

your code is working fine base on the program.

Answer 2

I'm just saying that, this code can get some errors depending of the input that you choose.
For example, try to put 0 as input. It will not work, but 0! = 1.