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=0,Stack[Maxn],top,num,head[Maxn],ans,id[Maxn];

void init()

{

    memset(index,-1,sizeof(index));

    memset(head,-1,sizeof(head));

    memset(vi,0,sizeof(vi));

    memset(low,0,sizeof(low));

    memset(dfn,0,sizeof(dfn));

    e=lab=top=num=ans=0;

}

struct Edge{

    int from,to,next,v;

}edge[Maxm];

void addedge(int from, int to)

{

    edge[e].v=0;

    edge[e].from=from;

    edge[e].to=to;

    edge[e].next=index[from];

    index[from]=e++;

    edge[e].v=0;

    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=0;

    edge[e].from=from;

    edge[e].to=to;

    edge[e].next=head[from];

    head[from]=e++;

    edge[e].v=0;

    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 0;

}

int dfs(int u)

{

    dfn[u]=low[u]=++lab;

    Stack[top++]=u;

    int i,j,temp;

    for(i=index[u];i!=-1;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^1].v=1;

        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 0;

}

int maxLen(int u)

{

    vi[u]=1;

    int i,j,temp=0,Max=0,lMax=0;

    for(i=head[u];i!=-1;i=edge[i].next)

    {

        if(!vi[edge[i].to])

        {

            temp=maxLen(edge[i].to);

            if(temp+1>=Max)

            {

                lMax=Max;

                Max=temp+1;

            }

            else

            {

                if(temp+1>lMax)

                    lMax=temp+1;

            }

            if(Max+lMax>ans)

                ans=Max+lMax;

        }

    }

    return Max;

}

int solve()

{

    int i,j,u,v,ed;

    dfs(1);

    ed=e;

    e=0;

    for(i=0;i<ed;i+=2)

    {

        u=id[edge[i].from];

        v=id[edge[i].to];

        add(u,v);

    }

    memset(vi,0,sizeof(vi));

    ans=0;

    maxLen(1);

    return num-1-ans;

}

int main()

{

    int m,i,j,a,b;

    while(scanf("%d%d",&n,&m),n||m)

    {

        init();

        for(i=1;i<=m;i++)

        {

            scanf("%d%d",&a,&b);

            addedge(a,b);

        }

        printf("%d\n",solve());

    }

    return 0;

}