Modified traversing printers to Iterable creators. Created null node factory.

[Beakerboy]

This PR changes method signatures, but does not break any tests. Instead of printing the tree, the preorder, etc method create an array in the specified order, which can be used for anything

Also created the __iter__() method.

instead of defining a null node within the RedBlackTree class, I moved to the Node class as a class method.

Let me know if you want anything tweaked.

All the next PRs I plan on submitting will be fully breaking tests. For example, I want to make the attributes private and create more setters and getters.

Questions for the next set of changes:

• is there a way to get rid of the Node.id attribute? It looks like the only time it is used is to demonstrate if two nodes are equal, which itself is only used to see if a particular node is null, correct? I’m thinking that setting Node.item to None would serve the same purpose.
• The self.item attribute is called “key” several places. I think it would be helpful to change it to self._key for consistency.
• Instead of having a self.size attribute, would it make sense to instead define __len__(self) and use that as the public facing interface? Internally, self._size Would still work.
• It would be nice to convert the print tree method to make use of the new traversal methods instead of having their own helper functions. Also, I’m guessing the other helper functions could be removed by moving everything into the preorder() etc methods using an optional method parameter.
• Are there any existing methods you think would be better as private? For example, the traversal helpers? If public, I think they should have a different name because they are not just helpers at that point, but part of the public interface.
• I want to allow the Node to have any data type as it’s key, not just an int. For example, a tuple can be used to carry out very complex sorting strategies, as is used In the native python array sort method. A complex custom object should be able to implement the Node interface and provide a tuple or some other “comparable” data type and be stored and retrieved from a tree.
• Instead of comparing Node.item in the tree object, we could implement __lt__(), __gt__(), etc and ask if node1 > node 2 instead of node1.item > node2.item
• a philosophical question (possibly addressed in your class)…. Do all null leaf node have to be identical? Currently a Node can have a null node to its left or right, but a null node has no parent. This means you can traverse down to a null node, but never up from one. I imagine from a memory management perspective, you save a lot of overhead if you have few objects in memory and just have multiple pointers to one object.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (34e52f6) 100.00% compared to head (0ff5b47) 100.00%.

Additional details and impacted files

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[Beakerboy]

Let me know if you want me to split this PR up into smaller units which would be faster to review.

Also, no need to answer my questions, they are just my ideas moving forward. If you have no strong opinions I can just do what I feel is most appropriate. If you turn on the discussions feature, I can post questions there instead of on a PR. Let me know what you prefer.

[emilydolson]

This looks good and I'm fine with questions in PRs! Sorry for the slow response - I had a bunch of work pile up.