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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.
It would be good to build the test cases up in terms of complexity i.e. input of "", then input of "1", then input of "1, 2" to let the users build up the algorithm in "true" TDD style.