1023: [SHOI2008]cactus仙人掌图

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
const int N=50000+5;
inline int read(){
int x=0;char ch;
while(ch<'0'||ch>'9')ch=getchar();
while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
return x;
}
struct Edge{int to,next;}e[N*20];
int f
,ans,que[N<<1],cir[N<<1],head
,cnt,fa
,dep
,lowlink
,pre
,dfs_clock;
void ins(int u,int v){
e[++cnt].to=v;e[cnt].next=head[u];head[u]=cnt;
}
void dp(int u,int root){
int n=dep[u]-dep[root]+1;
for(int i=u;i!=root;i=fa[i])
cir[n--]=f[i];
cir
=f[root];
n=dep[u]-dep[root]+1;
for(int i=1;i<=n;i++)cir[i+n]=cir[i];
que[1]=1;int head=1,rear=1;
for(int i=2;i<=n*2;i++){
while(head<=rear&&i-que[head]>n/2)head++;
ans=max(ans,i-que[head]+cir[i]+cir[que[head]]);
while(head<=rear&&cir[que[rear]]-que[rear]<=cir[i]-i)rear--;
que[++rear]=i;
}
for(int i=2;i<=n;i++)
f[root]=max(f[root],cir[i]+min(i-1,n-i+1));
}
void tarjan(int u){
lowlink[u]=pre[u]=++dfs_clock;
for(int i=head[u];i;i=e[i].next){
int v=e[i].to;
if(v==fa[u])continue;
if(!pre[v]){
fa[v]=u;dep[v]=dep[u]+1;
tarjan(v);
lowlink[u]=min(lowlink[u],lowlink[v]);
}else lowlink[u]=min(lowlink[u],pre[v]);
if(pre[u]<lowlink[v]){
ans=max(ans,f[u]+f[v]+1);
f[u]=max(f[u],f[v]+1);
}
}
for(int i=head[u];i;i=e[i].next){
int v=e[i].to;
if(fa[v]!=u&&pre[u]<pre[v])
dp(v,u);
}
}
int main(){
int n,m;n=read();m=read();
int k,u,v;
while(m--){
k=read();u=read();k--;
while(k--){
v=read();ins(u,v);ins(v,u);
u=v;
}
}
tarjan(1);
printf("%d",ans);
return 0;
}