* update doubly linked list
* reformat code
add more test
* add test to iter
* updating DIRECTORY.md
Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
* update doubly linked list
* reformat code
add more test
* add test to iter
* updating DIRECTORY.md
Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>