* luhn.py: Favor list comprehensions over maps
As discussed in CONTRIBUTING.md.
* updating DIRECTORY.md
Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
* Luhn algorithm
Perform Luhn validation on input string
Algorithm:
* Double every other digit starting from 2nd last digit.
* Subtract 9 if number is greater than 9.
* Sum the numbers
https://en.wikipedia.org/wiki/Luhn_algorithm
* Update DIRECTORY.md
* Update luhn.py
* Update luhn.py
* Update luhn.py
* Update luhn.py
* Update DIRECTORY.md