It will fail at the specific part where user code fails, though, so I don't think that's too much of a problem.

Yeah this was due to hidden tab characters, fixed now.

I agree. String ++ Nothing = Nothing or String? Both solutions seem to be possible. I guess this is gilgamec's trouble.

Well, after I read in description, that efficiency is important, I first thought of this. If it had not worked, I would have tried introselect :D

As far as I know, the Θ(n) solution has a very big constant. I'm not entirely sure how big it is but it may very well not be faster for any number of elements that can be tested on Codewars.

I tried to do it in Θ(n) :/

fixed! thanks :)

It's a part of the task.

It's a part of the task.

Good idea, I added a note

good idea, done!

You're right. I changed the description.
Now, it should be clearer.

You are right, I updated the Kata description. Thanks

Tests added.