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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.
Sorry to say, that's not accepted in a real interview. Your interviewer has given you the question to check whether you can whether you can solve it without using libraries. They expect you to understand how things work under the hood.