Update doubly linked list (#3619)

* 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>

Du Yuanchao committed 5y ago

5e642607c8958aa1e4277f75399442b2048a4725

Parent: 9b95e4f

Committed by GitHub <noreply@github.com> on 10/21/2020, 2:31:09 PM