test it with 999, thats a better test to do.

as in description: "The input will be a non-negative integer."

I find this one easy to read, short, idiomatic and clever.
The base case of the recursion is at first to it also saves valuable computing time when you reach the top of the stack.

It looks through the entire data set though, when the solution might be obvious after 3 reads. The original brief did state "The tests contain some very huge arrays, so think about performance.".

I find this idiomatic, understandable and clever. Should be on top of best practices.

I loved this solution!

I liked how you managed the upper/lower cases so simple, some here decided to loop again at the end

🖐👍

We thought the same solution! high five! =)

If you're wondering why I used threads for a simple solution... well... I had runtime error and wanted to experiment, this is my first multithreading script.