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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.
arr.length is a property of Array.prototype and the kata prohibited Array.prototype methods. And I agree with you, so many solutions are using temp array, the worse is that they are marked as clever...