bzoj千题计划224：bzoj1023: [SHOI2008]cactus仙人掌图

又写了一遍，发出来做个记录

#include<cstdio>
#include<algorithm>
#include<iostream>using namespace std;#define N 500001int tot=,front[N],to[N<<],nxt[N<<];int id,dfn[N],low[N];int fa[N],dep[N];int dp[N];
int tmp[N],q[N];int ans;void read(int &x)
{
    x=; char c=getchar();
    while(!isdigit(c)) c=getchar();
    while(isdigit(c)) { x=x*+c-''; 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 circle(int x,int y)
{
    int cnt=dep[y]-dep[x]+;
    int now=y;
    while(dfn[fa[now]]>=dfn[x]) tmp[cnt--]=now,now=fa[now];
    tmp[cnt]=now;
    cnt=dep[y]-dep[x]+;
    int nn=cnt;
    for(int i=;i<=cnt;++i) tmp[++nn]=tmp[i];
    int h=,t=;
    for(int i=;i<=nn;++i)
    {
        while(h<t && i-q[h]>cnt/) h++;
        if(h<t) ans=max(ans,dp[tmp[q[h]]]+dp[tmp[i]]+i-q[h]);
        while(h<t && dp[tmp[i]]-i>dp[tmp[q[t-]]]-q[t-]) t--;
        q[t++]=i;
    }
    for(int i=;i<=cnt;++i) dp[x]=max(dp[x],dp[tmp[i]]+min(i-,cnt-i+));
}void tarjan(int x,int y)
{
    dfn[x]=low[x]=++id;
    int t;
    for(int i=front[x];i;i=nxt[i])
    {
        if(i==(y^)) continue;
        t=to[i];
        if(!dfn[t])
        {
            fa[t]=x;
            dep[t]=dep[x]+;
            tarjan(t,i);
            low[x]=min(low[x],low[t]);
        }
        else low[x]=min(low[x],dfn[t]);
        if(dfn[x]<low[t])
        {
            ans=max(ans,dp[x]+dp[t]+);
            dp[x]=max(dp[x],dp[t]+);
        }
    }
    for(int i=front[x];i;i=nxt[i])
    {
        if(i==(y^)) continue;
        t=to[i];
        if(fa[t]!=x && dfn[x]<dfn[t]) circle(x,t);
    }
}int main()
{
    int n,m;
    read(n); read(m);
    int k,x,last;
    while(m--)
    {
        read(k);
        read(last);
        while(--k)
        {
            read(x);
            add(last,x);
            last=x;
        }
    }
    tarjan(,);
    printf("%d",ans);
}