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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.
Please don't spoil solutions, thanks. Take a pencil and a paper and check for this, it's pure logic.
You need to learn data structures, specifically LinkedList. Look for a course online, choosing a data structure to solve a problem is usually the most crucial step :)
Try another kata ;-)