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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.
This should have at least a Math tag, similar to http://www.codewars.com/kata/last-digit-of-a-huge-number
Agreed with @magthe. This question should be more clear. It should say what methods can/can't be called on the node class, how comparison of nodes should be done, and should explicitly state that hashing functions/data structures are not to be used (as opposed to saying you broke them -- I at first dismissed this as a random comment that was accidentally left in). None of these make the problem easier to solve, but they'd go a long way to making it less frustrating.