题目给的又不是有根树，你怎么能不建双向边啊。。。
by Islauso @ 2020-11-06 16:46:12
@[Mr_steamedbread](/user/177540)
by Islauso @ 2020-11-06 16:46:56
@[Segment_Tree_](/user/183235) %%%orz
by Mr_steamedbread @ 2020-11-06 16:56:20