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 = 0;
    memset(first,-1,sizeof(first));
    memset(fr,-1,sizeof(fr));
    memset(in,0,sizeof(in));
    memset(DFN,0,sizeof(DFN));
    memset(vis,0,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 = 1;
    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] = 1;
    for(i = first[u];i != -1;i = edge[i].next)
    {
        v = edge[i].v;
        if(vis[i]) continue;
        vis[i] = vis[i^1] = 1;
        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] = 0;
            cnum[scnt] ++;
        }while(u != v);
    }
}
int spfa(int x)
{
    int i,key,v,u,minz = 0;
    for(i = 1;i <= scnt;i ++)
    {
        in[i] = 0;
        d[i] = INF;
    }
    queue<int> que;
    in[x] = 1;
    d[x] = 0;
    que.push(x);
    while(!que.empty())
    {
        u = que.front();
        in[u] = 0;
        que.pop();
        for(i = fr[u];i != -1;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] = 1;
                    que.push(v);
                }
            }
        }
    }
    for(i = 1;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 == 0&&m == 0) break;
        CL();
        for(i = 1;i <= m;i ++)
        {
            scanf("%d%d",&u,&v);
            if(i == 1)
            str = u;
            qu[i] = u;
            qv[i] = v;
            add(u,v);
            add(v,u);
        }
        tot = 0;
        Tarjan(u,0);
        tot = 0;
        for(i = 1;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 == 1)
        printf("0
");
        else
        {
            x = spfa(spfa(1));
            printf("%d
",scnt-d[x]-1);
        }
    }
    return 0;
}