这更像是一个记录，而不是讲解#
P8817#
1. 边权为 1 的图，bfs 可做到 O (n)
2.a，d 容易确定先确定它们
P8818#
1. 分类讨论
P8819#
1. 可以反击的条件转换成所有点的出度为 1
2. 出度不好维护，转换成入度
3. 入读只能满足必要性，无法满足充分性 —— 随机的力量（看题解理解）
一些oi题目的关键思维突破点
AI-generated summary
The document discusses key insights for solving certain competitive programming problems (P8817, P8818, P8819). It highlights that for graphs with edge weights of 1, BFS can achieve O(n) complexity. It emphasizes the importance of determining specific points early on and suggests a classification discussion for problem P8818. For problem P8819, it notes that conditions for counterattacks can be transformed into ensuring all points have an out-degree of 1, but maintaining out-degrees is challenging, leading to a shift towards in-degrees. However, in-degrees only satisfy necessary conditions, not sufficient ones, indicating the need for randomization in understanding solutions.