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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.
O(1) is much better than O(n), good job all!
It's a tossup about whether to divide by 2 and then square or to square and then divide by 4. Since Ruby doesn't overflow, I picked the latter.