P7687 [CEOI 2005] Critical Network Lines

A communication network contains several nodes and several undirected **communication lines** that directly connect these nodes. The given network is connected, that is: for any pair of nodes, there exists a **communication path** formed by connecting (several) **communication lines** end to end. Some nodes provide service of type $A$ to all nodes (including themselves), and some nodes provide service of type $B$ to all nodes (including themselves). A node may provide both types of services. Every node must be able to access both services. When a **communication line** breaks, it may happen that some node cannot access a certain service. (That is, there exist a node and a service type such that there is no node that provides this service type and is connected to that node.) We call any **communication line** that can cause such a situation a **critical communication line**. Your task is to write a program to compute how many **critical communication lines** there are, and output the two endpoints of each **critical communication line**.

The first line contains four integers $N,M,K,L$. Here, $N \; (1 \le N \le 10^5)$ is the number of nodes in the network, $M \; (1 \le M \le 10^6)$ is the number of **communication lines**, $K \; (1 \le K \le N)$ is the number of nodes that provide service type $A$, and $L \; (1 \le L \le N)$ is the number of nodes that provide service type $B$. The nodes are numbered from $1$ to $N$. The second line contains $K$ integers, which are the node indices that provide service type $A$. The third line contains $L$ integers, which are the node indices that provide service type $B$. The next $M$ lines each contain two integers $p,q \; (1 \le p,q \le N,p \neq q)$, representing the indices of the two endpoints of a **communication line**. It is guaranteed that between any two nodes there is at most one **communication line**.

The first line contains an integer $S$, the number of **critical communication lines**. The next $S$ lines each contain two integers $p,q$, indicating the two endpoints of a **critical communication line**. The order of the **critical communication lines** can be arbitrary, and the order of the two endpoints for each **critical communication line** can also be arbitrary.

This problem is CEOI2005 D2T2. For the original statement, see: [Critical Network Lines](http://ceoi.inf.elte.hu/ceoi2005/download/tasks/day2/net.htm). Thanks to @[wsyhb](https://www.luogu.com.cn/user/145355) for providing the Special Judge. Translated by ChatGPT 5