Optimized O(N) -> O(1) for a __contains__ check inside linked_list/has_loop.py

[yoreei]

Describe your change:

data_structures/linked_list/has_loop.py: class Node: iter:
Previous implementation used lists for keeping track of visited nodes. The corrected version uses sets. See
https://wiki.python.org/moin/TimeComplexity

• Add an algorithm?
• Fix 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.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[algobytewise]

I think your comment in line 17 needs one more space before the #-sign if I read the error message correctly.

[yoreei]

@algobytewise You are right. I also accidentally pushed a separate PR into this one. I'll revert that and make it PEP8 conformant.

[cclauss]

What would be supercool is to have both approaches in a single file with a timeit (or similar) benchmark that quantifies the performance improvement.

[yoreei]

@cclauss That's a great idea. Thanks for bringing it up.

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

Please reopen this pull request once you commit the changes requested or make improvements on the code. If this is not the case and you need some help, feel free to seek help from our Gitter or ping one of the reviewers. Thank you for your contributions!