[noi2013]快餐店 基环树dp，单调队列维护最大值和次大值

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
#define N 220000
#define inf 0x3ffffffffffffffLL
typedef long long ll;
int v[N],e[N],ne[N],nn,w[N];
void add(int x,int y,int z){
    ne[++nn]=e[x],e[x]=nn,v[nn]=y,w[nn]=z;
}
int n;
int dfn[N];

int tot;
int fa[N],ww[N*2],f2[N];
int bb,b1[N*2];
ll ss[N*2];
void dfs(int x){
    dfn[x]=++tot;
    for(int i=e[x];i;i=ne[i]){
        if(fa[x]==v[i])continue;
        if(!dfn[v[i]]){
            fa[v[i]]=x;
            f2[v[i]]=w[i];
            dfs(v[i]);
        }else{
            if(dfn[v[i]]>dfn[x]){
                for(int j=v[i];j!=x;j=fa[j]){
                    b1[++bb]=j;
                    ww[bb]=f2[j];
                }
                b1[++bb]=x;
                ww[bb]=w[i];
            }
        }
    }
}
int been[N];
ll dp1[N],dp2[N];
ll ans2;
int q1[N*2],he1,bo1;
int q2[N*2],he2,bo2;
ll v1[N*2],v2[N*2];
void dfs2(int x,int fa){
    for(int i=e[x];i;i=ne[i]){
        if(fa==v[i]||been[v[i]])continue;
        dfs2(v[i],x);
        ans2=max(ans2,dp1[x]+dp1[v[i]]+w[i]);
        dp1[x]=max(dp1[x],dp1[v[i]]+w[i]);
    }
}
ll ma;
void gg(int x,int y){
    if(q1[x]!=q2[y]){
      ma=max(ma,v1[x]+v2[y]);    
    }
}
int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;i++){
        int a,b,c;
        scanf("%d%d%d",&a,&b,&c);
        add(a,b,c);
        add(b,a,c);
    }
    dfs(1);
    for(int i=1;i<=bb;i++)been[b1[i]]=1;
    for(int i=1;i<=bb;i++){
        dfs2(b1[i],0);
    }
  for(int i=1;i<=bb;i++)ww[i+bb]=ww[i],b1[i+bb]=b1[i];
  for(int i=1;i<=bb*2;i++)ss[i]=ss[i-1]+(ll)ww[i];
   ll ans=inf;
   q1[he1=bo1=1]=1;
   q2[he2=bo2=1]=1;
   q1[++he1]=2;
   q2[++he2]=2;
   for(int i=3;i<=bb*2;i++){
       if(i-q1[bo1]>=bb)bo1++;
       if(i-q2[bo2]>=bb)bo2++;
       ma=0;
       while(he1>bo1+1&&v1[he1]>v1[he1-1]){
           swap(v1[he1],v1[he1-1]);
           swap(q1[he1],q1[he1-1]);
           he1--;    
        }
       while(he2>bo2+1&&v2[he2]>v2[he2-1]){
          swap(v2[he2],v2[he2-1]);
          swap(q2[he2],q2[he2-1]);    
          he2--;
        }
       q1[++he1]=i;
       q2[++he2]=i;
       v1[he1]=dp1[b1[i]]+ss[i-1];
       v2[he2]=dp1[b1[i]]-ss[i-1];
       if(i>bb){
          gg(bo1,bo2);
          gg(bo1,bo2+1);
          gg(bo1,he2);
          gg(bo1+1,bo2);
          gg(bo1+1,bo2+1);
          gg(bo1+1,he2);
          gg(he1,bo2);
          gg(he1,bo2+1);
          ans=min(ans,ma);
       }
        
    }
    ans=max(ans,ans2);
  if(ans%2LL==0LL){
       printf("%lld.0",ans/2LL);    
    }else{
       printf("%lld.5",ans/2LL);    
    }
 
 }