.

now, click on the pseudo of the user and guess what happened to him... ;)

Hey pinkston. Here's a demonstration for why you usually don't want to mutate an input array:

randomArray = generateRandomTestCase()
assertEquals(Kata.solve(randomArray), Validator.solve(randomArray))

You can see that if you change the contents of "randomArray" in your solution, the validator that checks your solution won't be running on the same input. Since you never really know if the test case is made this way or not, it's just safe to assume that it is and keep the original array untouched.

Hope that resolves your confusion! :)

Unless mutating the input is a requirement, never mutate it. In a lot of katas (like this one), the input is passed to your function and after that, it's passed to a control function, so if you mutate it, wrong expected results will appear.

why I should not mutate an input in a situation like this

Because you never want to mutate the input unless you definitely know that it won't lead to any unexpected behavior.

maybe provide an example of when one should and shouldn't mutate an input

Don't mutate the input.

use rather something more appropriate... ;)

do I have to create a custom List class with a modified getitem method?

I really can tell much more without spoiling the solution, read what you have to find in the instructions, think about how are Fibonacci numbers changing and it'll come eventually.

Have you tried with a loop?