HDU 4612 Warm up(Tarjan)

果断对Tarjan不熟啊，Tarjan后缩点，求树上的最长路，注意重边的处理，借鉴宝哥的做法，开标记数组，标记自己的反向边。

 #pragma comment(linker, "/STACK:1024000000,1024000000")
 #include <iostream>
 #include <cstring>
 #include <cstdio>
 #include <queue>
 #include <cstdlib>
 using namespace std;
 #define N 200001
 #define M 2000001
 #define INF 0x3f3f3f3f
 struct node
 {
     int u,v,next;
 }edge[M];
 struct na
 {
     int u,v,w,next;
 }tree[M];
 int first[N],DFN[N],Belong[N],stack[N],Low[N],cnum[N];
 int in[N],d[N],fr[N];
 int qu[M],qv[M];
 int n,m;
 bool vis[M];
 int tot,scnt,top;
 void CL()
 {
     tot = scnt = top = ;
     memset(first,-,sizeof(first));
     memset(fr,-,sizeof(fr));
     memset(in,,sizeof(in));
     memset(DFN,,sizeof(DFN));
     memset(vis,,sizeof(vis));
 }
 void add(int u,int v)
 {
     edge[tot].u = u;
     edge[tot].v = v;
     edge[tot].next = first[u];
     first[u] = tot ++;
 }
 void addt(int u,int v)
 {
     tree[tot].u = u;
     tree[tot].v = v;
     tree[tot].w = ;
     tree[tot].next = fr[u];
     fr[u] = tot ++;
 }
 void Tarjan(int u,int num)
 {
     int v,i;
     DFN[u] = Low[u] = ++ tot;
     stack[top++] = u;
     in[u] = ;
     for(i = first[u];i != -;i = edge[i].next)
     {
         v = edge[i].v;
         if(vis[i]) continue;
         vis[i] = vis[i^] = ;
         if(!DFN[v])
         {
             //printf("cc%d %d",v,i);
             Tarjan(v,i);
             Low[u] = min(Low[u],Low[v]);
         }
         else if(in[v])
         {
             Low[u] = min(Low[u],DFN[v]);
         }
     }
     if(DFN[u] == Low[u])
     {
         scnt ++;
         do
         {
             v = stack[--top];
             Belong[v] = scnt;
             in[v] = ;
             cnum[scnt] ++;
         }while(u != v);
     }
 }
 int spfa(int x)
 {
     int i,key,v,u,minz = ;
     for(i = ;i <= scnt;i ++)
     {
         in[i] = ;
         d[i] = INF;
     }
     queue<int> que;
     in[x] = ;
     d[x] = ;
     que.push(x);
     while(!que.empty())
     {
         u = que.front();
         in[u] = ;
         que.pop();
         for(i = fr[u];i != -;i = tree[i].next)
         {
             v = tree[i].v;
             if(d[v] > d[u] + tree[i].w)
             {
                 d[v] = d[u] + tree[i].w;
                 if(!in[v])
                 {
                     in[v] = ;
                     que.push(v);
                 }
             }
         }
     }
     for(i = ;i <= scnt;i ++)
     {
         if(minz < d[i])
         {
             minz = d[i];
             key = i;
         }
     }
     return key;
 }
 int main()
 {

     int i,u,v,str,x;
     while(scanf("%d%d",&n,&m)!=EOF)
     {
         if(n == &&m == ) break;
         CL();
         for(i = ;i <= m;i ++)
         {
             scanf("%d%d",&u,&v);
             if(i == )
             str = u;
             qu[i] = u;
             qv[i] = v;
             add(u,v);
             add(v,u);
         }
         tot = ;
         Tarjan(u,);
         tot = ;
         for(i = ;i <= m;i ++)
         {
             if(Belong[qu[i]] != Belong[qv[i]])
             {
                 addt(Belong[qu[i]],Belong[qv[i]]);
                 addt(Belong[qv[i]],Belong[qu[i]]);
             }
         }
         if(scnt == )
         printf("0\n");
         else
         {
             x = spfa(spfa());
             printf("%d\n",scnt-d[x]-);
         }
     }
     return ;
 }