loj 1026( tarjan + 输出割边 ）

题目链接：http://lightoj.com/volume_showproblem.php?problem=1026

思路：Tarjan 算法简单应用。割边的特点:low[v]>dfn[u]（v是u的子节点)

 #include<iostream>
 #include<cstdio>
 #include<cstring>
 #include<algorithm>
 #include<vector>
 using namespace std;
 #define MAXN 22222

 struct Edge{
     int v,next;
 }edge[MAXN*];

 int n,NE;
 int head[MAXN];

 void Insert(int u,int v)
 {
     edge[NE].v=v;
     edge[NE].next=head[u];
     head[u]=NE++;
 }

 int cnt,bcc_count;
 int low[MAXN],dfn[MAXN];
 bool mark[MAXN];
 vector<pair<int,int> >bridge;

 void Tarjan(int u,int father)
 {
     int flag=;
     low[u]=dfn[u]=++cnt;
     mark[u]=true;
     for(int i=head[u];i!=-;i=edge[i].next){
         int v=edge[i].v;
         if(v==father&&!flag){ flag=;continue; }
         if(dfn[v]==){
             Tarjan(v,u);
             low[u]=min(low[u],low[v]);
             if(low[v]>dfn[u]){
                 bridge.push_back(make_pair(min(u,v),max(u,v)));
             }
         }else if(mark[v]){
             low[u]=min(low[u],dfn[v]);
         }
     }
 }

 int cmp(const pair<int,int>p,const pair<int,int>q)
 {
     if(p.first!=q.first)return p.first<q.first;
     return p.second<q.second;
 }

 int main()
 {
     int _case,u,v,x,t=;
     scanf("%d",&_case);
     while(_case--){
         scanf("%d",&n);
         NE=;
         memset(head,-,sizeof(head));
         for(int i=;i<n;i++){
             scanf("%d (%d)",&u,&x);
             while(x--){
                 scanf("%d",&v);
                 Insert(u,v);
             }
         }
         bridge.clear();
         cnt=bcc_count=;
         memset(dfn,,sizeof(dfn));
         memset(mark,false,sizeof(mark));
         for(int i=;i<n;i++)if(dfn[i]==)Tarjan(i,-);
         printf("Case %d:\n",t++);
         printf("%d critical links\n",(int)bridge.size());
         sort(bridge.begin(),bridge.end(),cmp);
         for(int i=;i<(int)bridge.size();i++){
             printf("%d - %d\n",bridge[i].first,bridge[i].second);
         }
     }
     return ;
 }