https://www.popit.kr/cors-preflight-%EC%9D%B8%EC%A6%9D-%EC%B2%98%EB%A6%AC-%EA%B4%80%EB%A0%A8-%EC%82%BD%EC%A7%88/
https://www.youtube.com/watch?v=HcppIYQLTxYv
https://medium.com/@junshengpierre/recursion-rockpaperscissors-js-b071a334080a
이진트리를 복기하였다.
https://www.youtube.com/watch?v=5cU1ILGy6dM
