Making the cache private is a bizzare "extra challenge". I can't see how that provides any better functionality/encapsulation, if anything it just makes it harder to understand.

Thank you for reply.

Ruby has the same "problem". However, a proper solution can be written in one line too.

On the other hand, this is an old kata, which are usually overrated compared to recent ones, and the difficulty cannot be changed anymore.

Take the easy points and carry on :-)

yes

You don't need convert sequince to list in start of your solution and to array in the end. Where and Sum functions work with arrays too.

I think that brute force solution is no good for 4 kyu task. It was good for 6 kyu task. IMHO