[ADD] create some files

[youngjinhan]

(Put an X inside the [ ] to denote check mark [X].)

• [X] If creating a new file :

  • [ ] added links to it in the README files ?
  • [X] included tests with it ?
  • [X] added description (overview of algorithm, time and space complexity, and possible edge case) in docstrings ?

• [ ] if done some changes :

  • [ ] wrote short description in the PR explaining what the changes do ?
  • [ ] Fixes #[issue number] if related to any issue

• [ ] other