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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.
Task doesn't mention constraints you are talking about, and it's still O(n).
This will traverse the list twice, which will lead to problems if used on rather large, but generated lists like
take 10000000 $ iterate f x.