hdu 4612 边连通度缩点+树的最长路径

思路：将以桥为分界的所有连通分支进行缩点，得到一颗树，求出树的直径。再用树上的点减去直径，再减一

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<map>
#include<queue>
#define Maxn 210110
#define Maxm 2501000
using namespace std;
int index[Maxn],vi[Maxn],dfn[Maxn],low[Maxn],e,n,lab=,Stack[Maxn],top,num,head[Maxn],ans,id[Maxn];
void init()
{
    memset(index,-,sizeof(index));
    memset(head,-,sizeof(head));
    memset(vi,,sizeof(vi));
    memset(low,,sizeof(low));
    memset(dfn,,sizeof(dfn));
    e=lab=top=num=ans=;
}
struct Edge{
    int from,to,next,v;
}edge[Maxm];
void addedge(int from, int to)
{
    edge[e].v=;
    edge[e].from=from;
    edge[e].to=to;
    edge[e].next=index[from];
    index[from]=e++;
    edge[e].v=;
    edge[e].to=from;
    edge[e].from=to;
    edge[e].next=index[to];
    index[to]=e++;
}
void add(int from,int to)
{
    edge[e].v=;
    edge[e].from=from;
    edge[e].to=to;
    edge[e].next=head[from];
    head[from]=e++;
    edge[e].v=;
    edge[e].to=from;
    edge[e].from=to;
    edge[e].next=head[to];
    head[to]=e++;
}
int Count(int u)
{
    ++num;
    int i;
    do
    {//将该连通分量进行标记
        i=Stack[--top];
        id[i]=num;
    }while(i!=u);
    return ;
}
int dfs(int u)
{
    dfn[u]=low[u]=++lab;
    Stack[top++]=u;
    int i,j,temp;
    for(i=index[u];i!=-;i=edge[i].next)
    {
        temp=edge[i].to;
        if(edge[i].v) continue;//一开始没加这个判断,一直WA
        edge[i].v=edge[i^].v=;
        if(!dfn[temp])
        {
            dfs(temp);
            low[u]=min(low[u],low[temp]);
        }
        low[u]=min(low[u],dfn[temp]);
    }
    if(dfn[u]==low[u])
        Count(u);
    return ;
}
int maxLen(int u)
{
    vi[u]=;
    int i,j,temp=,Max=,lMax=;
    for(i=head[u];i!=-;i=edge[i].next)
    {
        if(!vi[edge[i].to])
        {
            temp=maxLen(edge[i].to);
            if(temp+>=Max)
            {
                lMax=Max;
                Max=temp+;
            }
            else
            {
                if(temp+>lMax)
                    lMax=temp+;
            }
            if(Max+lMax>ans)
                ans=Max+lMax;
        }
    }
    return Max;
}
int solve()
{
    int i,j,u,v,ed;
    dfs();
    ed=e;
    e=;
    for(i=;i<ed;i+=)
    {
        u=id[edge[i].from];
        v=id[edge[i].to];
        add(u,v);
    }
    memset(vi,,sizeof(vi));
    ans=;
    maxLen();
    return num--ans;
}
int main()
{
    int m,i,j,a,b;
    while(scanf("%d%d",&n,&m),n||m)
    {
        init();
        for(i=;i<=m;i++)
        {
            scanf("%d%d",&a,&b);
            addedge(a,b);
        }
        printf("%d\n",solve());
    }
    return ;
}