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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.
Cool feedback. I'd be curious as to what sort of background someone might have where they are more familiar with data structure and algorithm puzzles than creating instances of a class with a default value in an OOP language.
As far as I know, the level of each kata is determined through feedback during its beta process, which for this particular one was more than 9 years ago. It's possible that criteria and other things have changed. The language versions and directions for testing, etc. seem to have.