like a bazooka instead of a nailgun :)

Fixed.

Your solution upvoted, clever (+1)

Yes, you're right. It's fixed. Thanks for your observation.

Let us condemn the moderator who approved this kata. :D

I think you may think that the ranking is determined by the author, in fact, on the contrary, it depends on the user's votes.

Many users tend to ignore the process of thinking when they vote.

Nice kata! Can you explain your formula?

I was surprised that this kata was ranked 6 kyu. If you happen to know the formula, and the only challenge is to translate the formula into code, then, yes, this is a very simple kata. But if you are starting from scratch, I think it deserves at least 5 kyu.

To be honest, this is not my original solution. I just copy this solution from somewhere ;-)
My original solution in JS version. Using this in python just because this one is easy to translate(no need change anything).

Thanks.
Might be you're right, because my version isn't fully optimal (see lechevalier's fork of my solution, for example). The idea with sets was to avoid to convert to string 2 times as you did, but that's more about playing with the concepts than efficiency.

If you still have your performance tests, it would be interesting to compare your solution with the one of lechevalier's fork ! :)

:]