1. [괄호 변환](https://school.programmers.co.kr/learn/courses/30/lessons/60058?language=java) 2. [캐시](https://school.programmers.co.kr/learn/courses/30/lessons/17680?language=java) 3. [뉴스 클러스터링](https://school.programmers.co.kr/learn/courses/30/lessons/17677?language=java)
