참고자료
1. https://ratsgo.github.io/machine%20learning/2017/03/26/tree/
2. https://leedakyeong.tistory.com/entry/%EC%9D%98%EC%82%AC%EA%B2%B0%EC%A0%95%EB%82%98%EB%AC%B4Decision-Tree-CART-%EC%95%8C%EA%B3%A0%EB%A6%AC%EC%A6%98-%EC%A7%80%EB%8B%88%EA%B3%84%EC%88%98Gini-Index%EB%9E%80
3. https://roytravel.tistory.com/105
4. https://data-science-hi.tistory.com/59