HDU 4612 Warm up（边双联通求树的直径）

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<vector>
#include<string>
#include<iostream>
#include<queue>
#include<cmath>
#include<map>
#include<stack>
#include<bitset>
using namespace std;
#define REPF( i , a , b ) for ( int i = a ; i <= b ; ++ i )
#define REP( i , n ) for ( int i = 0 ; i < n ; ++ i )
#define CLEAR( a , x ) memset ( a , x , sizeof a )
typedef long long LL;
typedef pair<int,int>pil;
const int maxn=200000+100;
const int maxm=3000000+100;
struct node{
    int to,next;
    bool col;//为桥
}e[maxm],e1[maxm];
int head[maxn],cnt,cntE,cnte;
int DFN[maxn],low[maxn];
int s[maxn],instack[maxn];
int idex,top,bridge;
int belong[maxn],h[maxn];
int d[maxn],vis[maxn];//为割;删除点后增加的联通快
int n,m;
void init()
{
    cnt=top=idex=0;
    cnte=bridge=cntE=0;
    CLEAR(head,-1);
    CLEAR(DFN,0);
    CLEAR(low,0);
    CLEAR(instack,0);
    CLEAR(belong,0);
    CLEAR(h,-1);
    CLEAR(vis,0);
    CLEAR(d,0);
}
void addedge(int u,int v)
{
    e[cntE].to=v;e[cntE].next=head[u];
    e[cntE].col=false;head[u]=cntE++;
}
void addedge1(int u,int v)
{
    e1[cnte].to=v;e1[cnte].next=h[u];
    h[u]=cnte++;
}
void Tarjan(int u,int pre)
{
    int v;
    low[u]=DFN[u]=++idex;
    s[top++]=u;
    instack[u]=1;
    int pre_num=0;
    for(int i=head[u];i!=-1;i=e[i].next)
    {
        v=e[i].to;
        if(v==pre&&!pre_num)
        {
            pre_num++;
            continue;
        }
        if(!DFN[v])
        {
            Tarjan(v,u);
            if(low[u]>low[v]) low[u]=low[v];
            if(low[v]>DFN[u])//桥
            {
                bridge++;
                e[i].col=true;
                e[i^1].col=true;
            }
        }
        else if(instack[v]&&low[u]>DFN[v])
            low[u]=DFN[v];
    }
    if(low[u]==DFN[u])
    {
        cnt++;
        do{
            v=s[--top];
            instack[v]=0;
            belong[v]=cnt;
        }while(v!=u);
    }
}
void dfs(int u,int depth)
{
    d[u]=depth;
    vis[u]=1;
    for(int i=h[u];i!=-1;i=e1[i].next)
    {
        int v=e1[i].to;
        if(!vis[v])
            dfs(v,depth+1);
    }
}
void work()
{
    REPF(i,1,n)
       if(!DFN[i])  Tarjan(i,-1);
    REPF(u,1,n)
        for(int i=head[u];i!=-1;i=e[i].next)
        {
            int v=e[i].to;
            if(belong[u]!=belong[v])
            {
                addedge1(belong[u],belong[v]);
                addedge1(belong[v],belong[u]);
            }
        }
    dfs(1,0);
    int pos=1;
    for(int i=1;i<=cnt;i++)
        if(d[i]>d[pos])  pos=i;
    CLEAR(vis,0);
    CLEAR(d,0);
    dfs(pos,0);
    int ans=0;
    REPF(i,1,cnt)  ans=max(ans,d[i]);
    printf("%d\n",bridge-ans);
}
int main()
{
    int u,v;
    while(~scanf("%d%d",&n,&m)&&(n+m))
    {
        init();
        REPF(i,1,m)
        {
            scanf("%d%d",&u,&v);
            if(u==v)  continue;
            addedge(u,v);
            addedge(v,u);
        }
        work();
    }
    return 0;
}