C

题意：有一些网络通过一些线路连接，求关键的连接，也就是桥，如果删除这个链接那么会产生两个子树

分析：注意一下图不是连通图即可

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#include<stdio.h>
#include<string.h>
#include<stack>
#include<algorithm>
using namespace std;

const int MAXN = 1005;

///构建邻接表
struct Edge{int v, next;}e[MAXN*MAXN];
int Head[MAXN], cnt;
void AddEdge(int u, int v)
{
    e[cnt].v = v;
    e[cnt].next = Head[u];
    Head[u] = cnt++;
}

///记录桥
struct Bridge{int u, v;}bri[MAXN];
int bnt;
bool cmp(Bridge n1, Bridge n2)
{
    if(n1.u != n2.u)
        return n1.u < n2.u;
    return n1.v < n2.v;
}

int f[MAXN];
int Dfn[MAXN], Low[MAXN], Index;

void InIt(int N)
{
    cnt = bnt = Index = 0;
    for(int i=0; i<=N; i++)
    {
        Head[i] = -1;
        Dfn[i] = false;
    }
}
void Tarjan(int u, int father)
{
    f[u] = father;
    Low[u] = Dfn[u] = ++Index;

    for(int j=Head[u]; j!=-1; j=e[j].next)
    {
        int v = e[j].v;
        if( !Dfn[v] )
        {
            Tarjan(v, u);
            Low[u] = min(Low[u], Low[v]);
        }
        else if( v != father )
            Low[u] = min(Low[u], Dfn[v]);
    }
}

int main()
{
    int N;

    while(scanf("%d", &N) != EOF)
    {
        int i, u, v, M;

        InIt(N);

        for(i=0; i<N; i++)
        {
            scanf("%d (%d)", &u, &M);

            while(M--)
            {
                scanf("%d", &v);
                AddEdge(u, v);
            }
        }

        for(i=0; i<N; i++)
        {
            if( !Dfn[i] )
                Tarjan(i, i);
        }

        for(i=0; i<N; i++)
        {
            int u = f[i];
            if( Low[i] > Dfn[u] )
            {
                bri[bnt].u = min(u, i);
                bri[bnt++].v = max(u, i);
            }
        }

        sort(bri, bri+bnt, cmp);

        printf("%d critical links
", bnt);

        for(i=0; i<bnt; i++)
            printf("%d - %d
", bri[i].u, bri[i].v);
        printf("
");
    }

    return 0;
}