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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.
Better late then never I suppose :) but here are a few hints:
Hint 1: A Consider fast way to find an index in an array
Hint 2: When you find the right index, it's a matter of splicing to get your solution
Hint 3: Remove Tree from your approach and you have the answer to hint #1