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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.
Certainly not coincidence, he is one of my students ;)
Try comparing this solution with https://www.codewars.com/kata/reviews/57a3503a7cfe50ebcb00014e/groups/58d061753f25db3d9600003f which is about 30% faster, although it still requires some attention to integer type choices.