相关链接
题目传送门：http://codeforces.com/contest/741/problem/D
中文题面：https://blog.sengxian.com/solutions/cf-741d
解题报告
看起来这个串的定义非常强的样子，但仔细观察不难发现，就是出现次数为奇数的字母最多出现一个
于是我们定时一个二进制状态$f_{i,j}$表示$i$到$j$这段路径中哪些字符出现了奇数次
我们考虑在每一条合法路径的LCA处将其统计
于是就变成了子树相关问题，于是非常自然想到启发式合并
考虑从子树最大的儿子那里继承集合，其他的儿子的集合暴力加入
因为走一条边，需要异或一个值，整个集合的转移我们可以记录一个标记，然后在插入时使其生效
考虑统计的话，我们在暴力插入的时候，枚举是哪一位不同，单次查询是$O(22)$的
又因为加入是$O(1)$的，所以总的时间复杂度$O(n \log n \cdot 22)$
好文章！666，学习了
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