Add generalized nCr (combinations) calculator for real n and integer r

[Naman-Vasudev]

Describe your change:

• Add an algorithm

This pull request adds a generalized nCr (combinations) calculator that works
for any real number n and non-negative integer r using the formula:

nCr = n * (n-1) * ... * (n-r+1) / r!

The implementation includes proper input validation, doctests for multiple
cases, and supports edge cases such as r=0.

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work.
• This PR only changes one algorithm file.
• All new Python files are placed inside an existing directory.
• All function parameters and return values are annotated with type hints.
• All functions have doctests that pass automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.

References

• Wikipedia: https://en.wikipedia.org/wiki/Binomial_coefficient

[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]

Variable and function names should follow the snake_case naming convention. Please update the following name accordingly: nCr

Please provide descriptive name for the parameter: n

Please provide descriptive name for the parameter: r