http://ybt.ssoier.cn:8088/problem_show.php?pid=1575
http://ybt.ssoier.cn:8088/problem_show.php?pid=1576
http://ybt.ssoier.cn:8088/problem_show.php?pid=1577
http://ybt.ssoier.cn:8088/problem_show.php?pid=1579
http://ybt.ssoier.cn:8088/problem_show.php?pid=1581
http://ybt.ssoier.cn:8088/problem_show.php?pid=1583
https://www.luogu.com.cn/problem/P2607 // 基环树dp