[mypy] Fix type annotations for linked_stack.py (#5576)

* Fix type annotations for linked_stack.py

* Replace Optional with inline union type

* Rename linked_stack to stack_with_singly_linked_list

* Rename stack_using_dll to stack_with_doubly_linked_list

* updating DIRECTORY.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

Edward Nuno committed 4y ago

94f38dd88c9f644d270a52f9cdd76c2e64e90c7c

Parent: 868c2fa

Committed by GitHub <noreply@github.com> on 10/31/2021, 4:03:03 PM