(连通图 模板题 无向图求桥)Critical Links -- UVA -- 796

链接：

https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=737

http://acm.hust.edu.cn/vjudge/contest/view.action?cid=82833#problem/C

说实话还不是太懂，自己再写点题理解理解！

代码：

 #include<cstdio>
 #include<cstdlib>
 #include<cmath>
 #include<iostream>
 #include<algorithm>
 #include<cstring>
 #include<vector>
 using namespace std;
 #define N 1005

 struct Edge{int v, next;}e[N*N];
 struct Bridge{int u, v;}bri[N];

 int bnt, n;
 int Head[N], cnt;
 int dfn[N], low[N], f[N], Index;

 bool cmp(Bridge a, Bridge b)
 {
     if(a.u != b.u)
         return a.u < b.u;
     return a.v < b.v;
 }

 void Init()
 {
     cnt = bnt = Index = ;
     memset(low, , sizeof(low));
     memset(dfn, , sizeof(dfn));
     memset(f, , sizeof(f));

     for(int i=; i<=n; i++)
     {
         Head[i] = -;
         dfn[i] = ;
     }
 }

 void AddEdge(int u, int v)
 {
     e[cnt].v = v;
     e[cnt].next = Head[u];
     Head[u] = cnt++;
 }

 void TarJan(int u, int fa)
 {
     f[u] = fa;
     low[u] = dfn[u] = ++Index;

     for(int j=Head[u]; j!=-; 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!=fa)
             low[u] = min(low[u], dfn[v]);
     }
 }

 int main()
 {
     while(scanf("%d", &n)!=EOF)
     {
         int i, j, u, v, m;

         Init();

         for(i=; i<n; i++)
         {
             scanf("%d (%d)", &u, &m);

             for(j=; j<m; j++)
             {
                 scanf("%d", &v);
                 AddEdge(u, v);
             }
         }

         for(i=; i<n; i++)
         {
             if(!dfn[i])
                 TarJan(i, i);
         }

         for(i=; 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\n", bnt);

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