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*22];

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=0;
    low[u]=dfn[u]=++cnt;
    mark[u]=true;
    for(int i=head[u];i!=-1;i=edge[i].next){
        int v=edge[i].v;
        if(v==father&&!flag){ flag=1;continue; }
        if(dfn[v]==0){
            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=1;
    scanf("%d",&_case);
    while(_case--){
        scanf("%d",&n);
        NE=0;
        memset(head,-1,sizeof(head));
        for(int i=0;i<n;i++){
            scanf("%d (%d)",&u,&x);
            while(x--){
                scanf("%d",&v);
                Insert(u,v);
            }
        }
        bridge.clear();
        cnt=bcc_count=0;
        memset(dfn,0,sizeof(dfn));
        memset(mark,false,sizeof(mark));
        for(int i=0;i<n;i++)if(dfn[i]==0)Tarjan(i,-1);
        printf("Case %d:
",t++);
        printf("%d critical links
",(int)bridge.size());
        sort(bridge.begin(),bridge.end(),cmp);
        for(int i=0;i<(int)bridge.size();i++){
            printf("%d - %d
",bridge[i].first,bridge[i].second);
        }
    }
    return 0;
}