luogu 4381 [IOI2008]Island 单调队列 + 基环树直径 + tarjan

Code:

#include<bits/stdc++.h>
using namespace std; 
#define ll long long 
#define maxn 1000006 
#define setIO(s) freopen(s".in","r",stdin) 
int n,edges,scc,num,ncc; 
int hd[maxn],to[maxn<<1],nex[maxn<<1]; 
long long val[maxn<<1];  
int low[maxn],dfn[maxn],vis[maxn],st[maxn],siz[maxn],idx[maxn];    
ll d1[maxn],d2[maxn];                          
long long sum[maxn<<1],dis[maxn<<1],dep[maxn<<1]; 
long long now=0;                                      
bool vis_edge[maxn<<1], vis_edge2[maxn<<1],mark[maxn];           
stack<int>S; 
deque<int>Q;  
struct Node 
{ 
    int cur; 
    long long val; 
    Node(int cur=0,ll val=0):cur(cur),val(val){}    
}nd[maxn<<1]; 
void addedge(int u,int v,int c) 
{
    nex[++edges]=hd[u],hd[u]=edges,to[edges]=v,val[edges]=1ll*c; 
}
void re() 
{
    for(int i=0;i<=num;++i) siz[i]=0;  
    scc=num=ncc=0;       
}
int get()
{
    return Q.size() ? Q.front() : 0; 
}
void tarjan(int u)
{
    low[u]=dfn[u]=++scc; 
    S.push(u); 
    vis[u]=1; 
    for(int i=hd[u];i;i=nex[i]) 
    {
        if(vis_edge[i]) continue; 
        vis_edge[i]=vis_edge[i^1]=1; 
        int v=to[i]; 
        if(!vis[v]) tarjan(v), low[u]=min(low[u],low[v]);      
        else if(vis[v]==1) low[u]=min(low[u], dfn[v]); 
    }
    if(low[u]==dfn[u]) 
    {
        ++num;  
        st[num]=u; 
        siz[num]=0;    
        for(;;)
        {
            int x=S.top();S.pop(); 
            ++siz[num];   
            vis[x]=-1; 
            idx[x]=num;           
            if(x==u) break; 
        }
    }
} 
void get_arr(int u) 
{
    for(int i=hd[u];i;i=nex[i]) 
    {
        int v=to[i]; 
        if(vis_edge2[i] || idx[v]!=idx[u]) continue;     
        vis_edge2[i]=vis_edge2[i^1]=1; 
        if(idx[v] == idx[u]) 
        {
            nd[++ncc]=Node(u, val[i]);   
            get_arr(v);  
            break;   
        }
    }     
}
ll tmp; 
void dfs(int u) 
{ 
    mark[u]=1;   
    d1[u]=d2[u]=0; 
    for(int i=hd[u];i;i=nex[i]) 
    {
        int v=to[i]; 
        if(mark[v]) continue;
        dep[v]=dep[u]+1ll*val[i];          
        dfs(v);         
        if(d1[v] + val[i] >= d1[u]) 
        {
            d2[u]=d1[u], d1[u]=d1[v] + val[i]; 
        }
        else if(d1[v] + val[i] > d2[u]) 
        {
            d2[u]=d1[v] + val[i]; 
        }
    }  
    now=max(now, dep[u]);  
    tmp=max(tmp, d1[u]+d2[u]); 
}
long long  solve(int root) 
{ 
    tarjan(root);
    for(int i=1;i<=num;++i) 
    {
        if(siz[i] > 1) 
        {
            get_arr(st[i]);
            break;  
        }
    }
    int k=ncc;  
    for(int i=1;i<=k;++i) nd[++ncc]=nd[i];      
    for(int i=1;i<=k;++i) mark[nd[i].cur]=1;  
    long long ans=0; 
    for(int i=1;i<=ncc;++i) 
    {
        tmp=0; 
        if(i>k) dis[i]=dis[i-k];      
        else dep[nd[i].cur]=now=0, dfs(nd[i].cur), dis[i]=now; 
        ans=max(ans, tmp);
        sum[i+1]=sum[i]+nd[i].val;    
        while(!Q.empty() && i - Q.front() >= k) Q.pop_front();   
        ans=max(ans,sum[i]+dis[i]+dis[get()]-sum[get()]);         
        while(!Q.empty() && dis[i]-sum[i]>=dis[Q.back()]-sum[Q.back()]) Q.pop_back(); 
        Q.push_back(i);   
    }
    sum[1]=0;          
    while(!Q.empty()) Q.pop_front();  
    re(); 
    return ans; 
}
int main()
{
    // setIO("input");  
    scanf("%d",&n);  
    edges=-1; 
    for(int i=1;i<=n;++i) 
    {
        int x,y; 
        scanf("%d%d",&x,&y); 
        addedge(i,x,y), addedge(x,i,y);      
    }
    long long ou=0; 
    for(int i=1;i<=n;++i) 
    {
        if(!vis[i]) { ou+=solve(i);   } 
    }
    printf("%lld\n",ou); 
    return 0; 
}