bzoj千题计划113：bzoj1023: [SHOI2008]cactus仙人掌图

#include<cstdio>
#include<iostream>
#include<algorithm>

using namespace std;

#define N 500001

int front[N],nxt[N<<2],to[N<<2],tot=1;

int dfn[N],low[N],fa[N],dep[N];

int dp[N],f[N],ans;

int tmp[N<<1],q[N];

void read(int &x)
{
    x=0; char c=getchar();
    while(!isdigit(c)) c=getchar();
    while(isdigit(c)) { x=x*10+c-‘0‘; c=getchar(); }
}

void add(int u,int v)
{
    to[++tot]=v; nxt[tot]=front[u]; front[u]=tot;
    to[++tot]=u; nxt[tot]=front[v]; front[v]=tot;
}

void circular(int x,int y)
{
    int cnt=dep[y]-dep[x]+1; int now=y;
    while(dfn[fa[now]]>=dfn[x]) tmp[cnt--]=now,now=fa[now];
    tmp[cnt]=now;
    cnt=dep[y]-dep[x]+1;
    int nn=cnt;
    for(int i=1;i<=cnt;++i) tmp[++nn]=tmp[i];
    int h=0,t=0;
    for(int i=1;i<=nn;++i)
    {
        while(h<t && i-q[h]>cnt/2) h++;
        if(h<t) ans=max(ans,dp[tmp[i]]+dp[tmp[q[h]]]+i-q[h]); 
        while(h<t && dp[tmp[i]]-i>dp[tmp[q[t-1]]]-q[t-1]) t--;
        q[t++]=i;
    }
    for(int i=2;i<=cnt;++i) dp[x]=max(dp[x],dp[tmp[i]]+min(i-1,cnt-i+1));
}

void tarjan(int x,int y)
{
    low[x]=dfn[x]=++tot;
    for(int i=front[x];i;i=nxt[i])
    {
        if(i==(y^1)) continue;
        if(!dfn[to[i]]) 
        {
            fa[to[i]]=x;
            dep[to[i]]=dep[x]+1;
            tarjan(to[i],i);
            low[x]=min(low[x],low[to[i]]);
        }
        else low[x]=min(low[x],dfn[to[i]]);
        if(dfn[x]<low[to[i]])
        {
            ans=max(ans,dp[x]+dp[to[i]]+1);
            dp[x]=max(dp[x],dp[to[i]]+1);
        }
    }
    for(int i=front[x];i;i=nxt[i])
    {
        if(i==(y^1)) continue;
        if(fa[to[i]]!=x && dfn[x]<dfn[to[i]]) circular(x,to[i]);
    }
}

int main()
{
    //freopen("bzoj_1023.in","r",stdin);
    //freopen("bzoj_1023.out","w",stdout);
    int n,m;
    read(n); read(m);
    int k,x,last;
    while(m--)
    {
        read(k); read(last);
        k--;
        while(k--) { read(x); add(x,last);    last=x;}
    }
    tot=0;
    tarjan(1,0);
    cout<<ans;
}