jajajajaajajaja u make my day

Most of the people who write the solution in a 8kyu kata are new programmers, many of them don't even know what O notation is, I guess it's the highest ranked because it's easy to understand and easy to think about it. Kata tests do not measure performance, so it is not a flag. I agree with the down buttons. but i think that the kata tests will have any performance input like a lot of inputs or timing. I submit the sort version to, because is the faster way to write the solution, are only to instructions.

This won't return an empty array if numbers is empty or nil.

Is it possible that this won't return an empty array if numbers is empty or nil?

yes, and also in some case the word sum will be equals but the words are very diferents. I note it after send the response. how i can modify the test cases of the kata ?

test case : "abc" and ["bbb"]

+Clever

From kata description:

an array of numbers in which two numbers occur once and the rest occur only twice.

this only work if the items array repeat only once, you can explain how will work for more thant once repeat.