binary_tree_traversals.py: Simplify with dataclasses

[cclauss]

Describe your change:

• Add an algorithm?
• Optimize a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
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[dhruvmanila]

Awesome work!

This is out of scope for this PR but just wanted to point out that I see a lot of Node classes containing a left and right node. I don't think that makes sense.

In case of tree structures a node can only contain key and data, key to identify the node and data is the containing information. The tree has left and right subtrees and not the node.

[dhruvmanila]

I did not notice this the first time but shouldn't this be Optional["Node"]?

[cclauss]

My sense was that if neither Python nor mypy was complaining about it then I would not either. This would be the only location where we would set these values to None unless we started deleting Nodes which I assumed we were not gonna do.

[dhruvmanila]

Python is never going to complain as it simply does not care about type hints. mypy will complain when you run it in a bit stricter modes. Plain mypy will not complain about a lot of stuff as that's how gradual typing goes.

And no, as per this definition, the leaves of the trees (the last nodes) will have its left and right node None as there are no left and right child (we have reached the end).

[cclauss]

This would be the only location where we would set these values to None

[dhruvmanila]

Yes, but that still won't allow the file to be type checked later on. The variable's correct type is Optional["Node"]. Just because it is being set None once, does not change the type of that variable. That's what 'static' means in static typing :)