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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
Probably it's not O(n) as you think.
Could you be more specific? If test cases were wrong, there would be many issues open, but there aren't.
What language are you using? Which input fails? And what do you think it should return?