相关链接
题目传送门：http://www.lydsy.com/JudgeOnline/problem.php?id=2001
神犇题解Ⅰ：http://www.cnblogs.com/BLADEVIL/p/3512644.html
神犇题解Ⅱ：http://blog.miskcoo.com/2015/04/bzoj-2001
解题报告
如果没有边权，只需要维护连通性
那么搞一个可以撤销的并查集就可以啦！
现在考虑带边权的情况，我们仍然对时间进行分治
在每一层，我们进行两个操作：
1. Contraction 缩小点的规模
将所有当前分治区间内会被修改的边的边权置为 $-INF$
然后做一遍MST，将此时通过非 $-INF$ 连接的点用并查集缩起来
因为$-INF$边只会有区间长度个，所以每一次分治后，点的规模不会超过分治区间的长度2. Reduction 缩小边的规模
将会被修改的边的边权置为 $INF$，然后再做一遍MST
将此时不在MST中的边删去，因为保留的边数不会超过点数，所以边的规模也不会超过分治区间的长度
所以时间复杂度写出来长这样： $T(n) = 2T(\frac{n}{2}) + n\log n$
用主定理化简之后，时间复杂度就是： $O(n{\log ^2}n)$
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