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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.
I think this is more of a Math kata than a programming kata, especially since iterative solutions are blocked out.
The tests seems to have timing issues. I retried a couple of times and finally it passed. I then discovered that the other solutions had the same or similar (AFAIK) time complexity.