https://toramko.tistory.com/entry/python-oslistdir%EA%B3%BC-oswalk-%ED%8C%8C%EC%9D%B4%EC%8D%AC-%ED%8A%B9%EC%A0%95-%EA%B2%BD%EB%A1%9C-%EB%82%B4-%EB%94%94%EB%A0%89%ED%86%A0%EB%A6%AC%EC%99%80-%ED%8C%8C%EC%9D%BC-%EA%B2%80%EC%83%89#google_vignette
os.listdir(): 1-depth 순회 os.walk(): 모든 depth 순회
