Best in terms of function operating time. I mean, why do complex calculations when you can do it straight away? As a complete beginner, I discover on other katas that sometimes the simplest head-on solutions are the best in time, but sometimes complex calculations work tens of times faster than similar head-on solutions. I would also do it through a set so that the algorithmic time is O(1)

Very smart

Nice and neat.

I'm working on learning regex and this was super helpful, thank you. It took me a little while to figure out how it worked, but it's slick.

Thanks

Very clean solution

Why? Is this a piece of gospel taken straight out of a "Clean Code" type book?
This code works and looks great.