Add longest common substring

[RohitSingh107]

Describe your change:

• 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}.

[algorithms-keeper]

Click here to look at the relevant links ⬇️

🔗 Relevant Links

Repository:

• Contributing guidelines
• Project Euler solution guidelines

Python:

• Formatted string literals (f-strings)
• Type hints
• doctest
• unittest
• pytest

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[algorithms-keeper]

Please provide return type hint for the function: longest_common_substring. If the function does not return a value, please provide the type hint as: def function() -> None:

[ChrisO345]

What happens if an invalid input is used? For example: longest_common_substring(1, 2)?

[RohitSingh107]

It will throw error as type hints say it should be a string.

[ChrisO345]

That's not the reason why an error is thrown. Type hints are used to make code more readable. Python is statically typed so the code runs and an error is thrown when it attempts to perform len() on an integer. https://www.geeksforgeeks.org/type-hints-in-python/

[RohitSingh107]

so what should I do? adding an assert that it should be a string?

[RohitSingh107]

Why pr is not progressing, is it something that needs to change or be modified?

[CaedenPH]

@cclauss Can we get your review please

[RohitSingh107]

Does this come from the second solution of https://www.geeksforgeeks.org/longest-common-substring-dp-29 Given the test data includes that site, it probably does. The source or inspiration should be acknowledged in the comments.

No, it's my original work, I haven't even seen this algorithm before writing on my own. I have only just taken test cases to test my algorithm and nothing else.

out of date

[poyea]

Thank you for your pull request!🤩