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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.
Most likely because whoever wrote this didn't realize it can be solved in O(1). Or because this kata is listed in the section "LINQ excercises" so the author tried to solve it using System.Linq library. I don't think it can be solved in O(1) using language integrated query.
Same here, that is a bit inexpected tbh.