poj 3177 边双联通 **

题意：给定一个连通的无向图G，至少要添加几条边，才能使其变为双连通图。

链接：点我

kuangbin模板题，分析链接：点我

 #include <stdio.h>
 #include <string.h>
 #include <iostream>
 #include <algorithm>
 #include <map>
 using namespace std;
 const int MAXN = ;//点数
 const int MAXM = ;//边数，因为是无向图，所以这个值要*2
 struct Edge
 {
 int to,next;
 bool cut;//是否是桥标记
 }edge[MAXM];
 int head[MAXN],tot;
 int Low[MAXN],DFN[MAXN],Stack[MAXN],Belong[MAXN];//Belong数组的值是1~block
 int Index,top;
 int block;//边双连通块数
 bool Instack[MAXN];
 int bridge;//桥的数目
 void addedge(int u,int v)
 {
     edge[tot].to = v;edge[tot].next = head[u];edge[tot].cut=false;
     head[u] = tot++;
 }
 void Tarjan(int u,int pre)
 {
     int v;
     Low[u] = DFN[u] = ++Index;
     Stack[top++] = u;
     Instack[u] = true;
     for(int i = head[u];i != -;i = edge[i].next)
     {
         v = edge[i].to;
         if(v == pre)continue;
         if( !DFN[v] )
         {
             Tarjan(v,u);
             if( Low[u] > Low[v] )Low[u] = Low[v];
             if(Low[v] > DFN[u])
             {
                 bridge++;
                 edge[i].cut = true;
                 edge[i^].cut = true;
             }
         }
         else if( Instack[v] && Low[u] > DFN[v] )
         Low[u] = DFN[v];
     }
     if(Low[u] == DFN[u])
     {
         block++;
         do
         {
             v = Stack[--top];
             Instack[v] = false;
             Belong[v] = block;
         }
         while( v!=u );
     }
 }
 void init()
 {
     tot = ;
     memset(head,-,sizeof(head));
 }
 int du[MAXN];//缩点后形成树，每个点的度数
 void solve(int n)
 {
     memset(DFN,,sizeof(DFN));
     memset(Instack,false,sizeof(Instack));
     Index = top = block = ;
     Tarjan(,);
     int ans = ;
     memset(du,,sizeof(du));
     for(int i = ;i <= n;i++)
         for(int j = head[i];j != -;j = edge[j].next)
             if(edge[j].cut)
                 du[Belong[i]]++;
     for(int i = ;i <= block;i++)
         if(du[i]==)
             ans++;
     //找叶子结点的个数ans,构造边双连通图需要加边(ans+1)/2
     printf("%d\n",(ans+)/);
 }
 int main()
 {
     int n,m;
     int u,v;
     while(scanf("%d%d",&n,&m)==)
     {
         init();
         while(m--)
         {
             scanf("%d%d",&u,&v);
             addedge(u,v);
             addedge(v,u);
         }
         solve(n);
     }
     return ;
 }