POJ——T2117 Electricity

 http://poj.org/problem?id=2117

Time Limit: 5000MS		Memory Limit: 65536K
Total Submissions: 5459		Accepted: 1788

 当m==0 时，特判输出 n-1~~~

先求出原始的连通分量，

然后只有删割点才会增加连通分量，枚举每个割点

最后加上原始的

#include <algorithm>
#include <cstring>
#include <cstdio>

using namespace std;

const int N(100015);
int n,m,u,v;
int sumedge,head[N<<1];
struct Edge
{
    int to,next;
    Edge(int to=0,int next=0) :
        to(to),next(next) {}
}edge[N*5];

void ins(int from,int to)
{
    edge[++sumedge]=Edge(to,head[from]);
    head[from]=sumedge;
}

int low[N<<1],dfn[N<<1],tim;
int sumcol,num[N];
int cutpoint[N];

void DFS(int now,int pre)
{
    low[now]=dfn[now]=++tim;
    int sumtredge=0,if_cutpoint=0;
    for(int i=head[now];i!=-1;i=edge[i].next)
    {
        int go=edge[i].to;
        if((i^1)!=pre)
        {
            if(!dfn[go])
            {
                sumtredge++;
                DFS(go,i);
                if(low[go]>=dfn[now])
                {
                    if_cutpoint=1;
                    num[now]++;
                }
                
                low[now]=min(low[now],low[go]);
            }
            else low[now]=min(low[now],dfn[go]);
        }
    }
    if(pre==-1)
    {
        if(sumtredge>1)  cutpoint[now]=1;
    }
    else if(if_cutpoint) cutpoint[now]=1;
}

int ans,cnt,tmp,vis[N],root[N],cut[N];

void link(int x)
{
    vis[x]=1;
    for(int i=head[x];i!=-1;i=edge[i].next)
    {
        v=edge[i].to;
        if(!vis[v]) link(v);
    }
}

void cut_point(int pre,int fa)
{
    vis[pre]=1;
    for(int i=head[pre];i!=-1;i=edge[i].next)
    {
        int go=edge[i].to;
        if(!vis[go])
        {
            if(pre==fa) cnt++;
            cut_point(go,fa);
        }
    }
}

void init(int n)
{
    sumedge=-1; ans=tim=tmp=cnt=0;
    memset(vis,0,sizeof(vis));
    memset(low,0,sizeof(low));
    memset(dfn,0,sizeof(dfn));
    memset(num,0,sizeof(num));
    memset(cut,0,sizeof(cut));
    memset(root,0,sizeof(root));
    memset(head,-1,sizeof(head));
    memset(cutpoint,0,sizeof(cutpoint));
}

int main()
{
//    freopen("makerout.txt","r",stdin);
//    freopen("myout.txt","w",stdout);
    
    while(~scanf("%d%d",&n,&m)&&n)
    {
        init(n);
        if(m==0) printf("%d
",n-1);
        else
        {
            for(;m;m--)    
            {
                scanf("%d%d",&u,&v);
                ins(u,v); ins(v,u);
            }
            for(int i=0;i<n;i++)
                if(!vis[i]) ++tmp,link(i);
            for(int i=0;i<n;i++)
                if(!dfn[i]) DFS(i,-1);
            for(int i=0;i<n;i++)
                if(cutpoint[i])
                {
                    memset(vis,0,sizeof(vis));
                    cut_point(i,i);
                    ans=max(ans,cnt-1);
                    cnt=0;
                }
            ans+=tmp;
            printf("%d
",ans);
        }
    }
    return 0;
}