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.

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.