코딩테스트 https://velog.io/@alex01/%EC%95%BD%EC%88%98%EC%9D%98-%ED%95%A9
클래스 https://velog.io/@alex01/Class-%EA%B0%9D%EC%B2%B4-Instance-%EA%B0%9D%EC%B2%B4
https://velog.io/@alex01/Class-self-init-%EB%A9%94%EC%84%9C%EB%93%9C
https://velog.io/@alex01/Class-%EC%83%81%EC%86%8D
