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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 beneficial to specify what sort of tree traversal should be used when flattening the structure.
Actually, the question could also become much more informative if you have a few separate functions that specify flattening via preorder, inorder, and postorder.