Could you explain an efficient solution which avoids doing this?

I've just wrote them an email with this idea=).

That's actually a much better idea, and should really be trivial to add.
You could even I think quite easily add statistics like speed percentile for the given language. That would make unusually slow solutions stand out and lead people to question why that might be.

That being said, I don't think this site sees a lot of development so this might be a fool's hope :P

Just because they're done easily doesn't mean they're the best. This makes multiple passes over an array

Or as an option, developers of this web site will have add something like load tests and show avg execution time near each sulution in such test. to give developerse oppotuniti competting in whose code is the most effective. And also add the fastest code filter in here. And the same to memory usage. Also such statistic may be useful to newbe.

Feels like the "Best Practices" button should be restricted or disabled entirely, since a whole bunch of people will upvote anything that's crammed into a neat one-liner as "Best Practices" even if it's horrendously inefficient.
Not that my code is exactly brilliant, but from the comments on a lot of these I get the feeling that many people are learning terrible habits.

O(n)+O(n) complexity?

I love seeing these done so easily after I just took 6 lines of code to complete the same task. (this was even after cleaning it up).

I did the same in the biginning, but the program didn't pass me

shut up giabanga you level 6 peasant

Expected and actual were flipped, fixed.

After I did my solution I saw this solution and a was just like "omg so easy"... Nice and really "Best practice"

same problem.

This looks nice and short but it iterates through the array twice, which is unnecessary.
I wouldn't really call this "best practice".