bzoj 1023 [SHOI2008]cactus仙人掌图 ( poj 3567 Cactus Reloaded )——仙人掌直径模板

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
const int N=5e4+5;
int n,m,hd[N],xnt,dfn[N],low[N],tim,fa[N],f[N],a[N<<1],q[N<<1],ans;//N<<1!!
struct Ed{
  int nxt,to;Ed(int n=0,int t=0):nxt(n),to(t) {}
}ed[N<<2];//<<2(?)
void add(int x,int y)
{
  ed[++xnt]=Ed(hd[x],y);hd[x]=xnt;
}
void dp(int x,int y)
{
  int m,i,l,r,p;
  for(m=0;y!=x;y=fa[y])a[++m]=f[y];// a is value
  for(a[++m]=f[x],i=1;i<m;i++)a[m+i]=a[i];// 2*m-1 is enough
  l=r=q[1]=1;p=(m>>1);
  for(i=2;i<=m+p;i++)
    {
      while(l<=r&&i-q[l]>p)l++;
      ans=max(ans,a[q[l]]+a[i]+i-q[l]);// a[],not f already(i is pos on a[])
      while(l<=r&&a[i]>=a[q[r]]+i-q[r])r--;
      q[++r]=i;
    }
  for(int i=1;i<m;i++)f[x]=max(f[x],a[i]+min(i,m-i));
}
void tarjan(int cr)
{
  dfn[cr]=low[cr]=++tim;
  for(int i=hd[cr],v;i;i=ed[i].nxt)
    if((v=ed[i].to)!=fa[cr])//!
      if(!dfn[v])
    {
      fa[v]=cr;tarjan(v);
      low[cr]=min(low[cr],low[v]);
      if(low[v]>dfn[cr])
        {
          ans=max(ans,f[cr]+f[v]+1);
          f[cr]=max(f[cr],f[v]+1);
        }
    }
      else low[cr]=min(low[cr],dfn[v]);
  for(int i=hd[cr],v;i;i=ed[i].nxt)
    if(fa[v=ed[i].to]!=cr&&dfn[v]>dfn[cr])dp(cr,v);
}
int main()
{
  scanf("%d%d",&n,&m);int k,x,y;
  for(int i=1;i<=m;i++)
    {
      scanf("%d%d",&k,&x);
      for(int j=1;j<k;j++)
    {
      scanf("%d",&y);
      add(x,y);add(y,x);x=y;
    }
    }
  tarjan(1);
  printf("%d",ans);
  return 0;
}