🆕🎓 Revisit tree node's having reference to their parents

[jameshughes89]

Related Issues or PRs

As discussed in #347.

What

Some of the binary tree functions can be simplified, or at least cleaned up, if each node has reference to their parents.

Why

Sure, it's not needed, but it would be cool to show an alternative implementation with a quick change to the node class. Besides, not having a reference to the parent is ultimately arbitrary.

Where

Topic24? Hard to Say. It could be an aside for topic 24, or perhaps something to discuss in 2nd year. Could always have fun and have it be an exam question or something.

How

Re-implement the binary tree node with a reference for a parent node.

Additional Notes

Obviously it's not needed, but it would be a great exercise for students either way.