记新生赛（2013多校第二场题解）

Code
#pragma comment(linker,"/STACK:102400000,102400000")
#include <iostream>
#include <cstdio>
#include <cstring>
#include <vector>
#define PII make_pair
#define edge vector< pair<int,int> >
#define fill(x,y) memset(x,y,sizeof(x));
 
using namespace std;

const int maxn=200000+10,maxm=1000000+10;
edge G[maxn];
int DFN[maxn],Low[maxn];
bool bridge[maxm];
int Index,ans,all,n,m;

void Tarjan(int u,int id)
    {
        DFN[u]=Low[u]=++Index;
        for(edge::iterator i=G[u].begin();i!=G[u].end();i++){
            int v=i->first,t=i->second;
            if(t==id) continue;
            if(!DFN[v]){
                Tarjan(v,t);
                Low[u]=min(Low[v],Low[u]);
                if(Low[v]==DFN[v]){
                    all+=(int)(!bridge[t]);
                    bridge[t]=1;
                }      
            }
              else Low[u]=min(DFN[v],Low[u]);
        }
    }
void dfs(int u)
    {
        DFN[u]=1;
        for(edge::iterator i=G[u].begin();i!=G[u].end();i++)
            if(Low[i->first]<0){
                Low[i->first]=Low[u]+(int)bridge[i->second];
                dfs(i->first);
            }    
    }
int main()
{
    while(cin>>n>>m){
        if((n==0)&&(m==0))break;
        for(int i=1;i<=n;i++)G[i].clear();
        fill(Low,0);
        fill(DFN,0);
        fill(bridge,0);
        Index=0;ans=0;all=0;
        for(int i=1;i<=m;i++){
            int x,y;
            cin>>x>>y;
            G[x].push_back(PII(y,i));
            G[y].push_back(PII(x,i));
        }
        for(int i=1;i<=n;i++)
            if(!DFN[i])Tarjan(i,0);
        fill(DFN,0);
        for(int i=1;i<=n;i++)
            if(!DFN[i]){
                fill(Low,255);Low[i]=0;dfs(i);
                int langest=0,s=0;
                for(int i=1;i<=n;i++)
                    if(langest<Low[i]){
                        langest=Low[i];
                        s=i;
                    }
                fill(Low,255);Low[s]=0;dfs(s);
                for(int i=1;i<=n;i++)
                    ans=max(ans,Low[i]);
            }
        cout<<all-ans<<endl;   
    }
    return 0;
}