POJ 2117 Electricity

#include <cstring>
#include <ctype.h>
#include <cstdio>
#define N 20005
void read(int &x)
{
    x=0;char ch=getchar();
    for(;!isdigit(ch);ch=getchar());
    for(;isdigit(ch);ch=getchar()) x=x*10+ch-‘0‘;
}
struct Edge
{
    int next,to;
    Edge (int next=0,int to=0) :next(next),to(to) {}
}edge[N<<1];
bool vis[N],cutpoint[N];
int sumcol,ans,head[N],cnt=-1,n,m,dfn[N],low[N],tim;
void init()
{
    memset(head,-1,sizeof(head));
    cnt=-1;
    memset(edge,0,sizeof(edge));
    ans=0;
    sumcol=0;
    tim=0;
    memset(dfn,0,sizeof(dfn));
    memset(vis,0,sizeof(vis));
    memset(low,0,sizeof(low)); 
}
void insert(int u,int v)
{
    edge[++cnt]=Edge(head[u],v);
    head[u]=cnt;
}
int max(int a,int b) {return a>b?a:b;}
int min(int a,int b) {return a>b?b:a;}
void tarjan(int x,int pre)
{
    vis[x]=1;
    low[x]=dfn[x]=++tim;
    int sum=0;
    int tot=0;
    for(int u=head[x];u!=-1;u=edge[u].next)
    {
        if(!vis[edge[u].to])
        {
            sum++;
            tarjan(edge[u].to,x);
            low[x]=min(low[x],low[edge[u].to]);
            if(low[edge[u].to]>=dfn[x]) tot++;
        }
        else if(dfn[edge[u].to]<dfn[x]&&edge[u].to!=pre) low[x]=min(low[x],dfn[edge[u].to]);
    }
    if(pre==-1) {if(sum>1) ans=max(ans,sum-1);}
    else ans=max(ans,tot);
}
int main()
{
    while(scanf("%d%d",&n,&m)!=EOF&&(n||m))
    {
        init();
        if(m==0)
        {
            printf("%d\n",n-1);
            continue;
        }
        for(int x,y;m--;)
        {
            read(x);
            read(y);
            insert(x,y);
            insert(y,x);
        }
        for(int i=0;i<n;i++)
        {
            if(!vis[i]) 
            {
                sumcol++;
                tarjan(i,-1);
            }
        }
        printf("%d\n",sumcol+ans);
    }
    return 0;
}