難問題系統をやる意味 https://www.ryoegami.com/blog/%e9%9b%a3%e5%95%8f%e9%a1%8c%e7%b3%bb%e7%b5%b1%e3%82%92%e3%82%84%e3%82%8b%e6%84%8f%e5%91%b3/