模板：缩点

翻之前记录翻出来缩点的板子

正好今天语文课看tarjan睡着了……

https://www.luogu.org/problemnew/show/P3387

#include<iostream>
#include<cstdio>
#include<queue>
using namespace std;
const int maxn=10000+10;
int n, m, cnt, num, s, top;
int v[maxn], dfn[maxn], low[maxn], sd[maxn];
int st[maxn], vis[maxn], d[maxn];
int h[maxn], head[maxn], in[maxn];
struct qwq{
    int to, next, from;
}edge[maxn*10], ed[maxn*10];
void add(int x,int y){
    edge[++cnt].next=head[x];
    edge[cnt].from=x;
    edge[cnt].to=y;
    head[x]=cnt;
}
void tarjan(int x){
    dfn[x]=low[x]=++num;
    st[++top]=x;
    vis[x]=1;
    for(int i=head[x];i;i=edge[i].next){
        int v=edge[i].to;
        if(!dfn[v]){
            tarjan(v);
            low[x]=min(low[x],low[v]);
        }
        else if(vis[v]){
            low[x]=min(low[x],dfn[v]);
        }
    }
    if(dfn[x]==low[x]){
        int y;
        while(y=st[top--]){
            sd[y]=x;
            vis[y]=0;
            if(x==y) break;
            v[x]+=v[y];
        }
    }
}
int toposort(){
    queue <int> q;
    int tot=0;
    for(int i=1; i<=n; i++)
        if(sd[i]==i && !in[i]){
        q.push(i);
        d[i]=v[i];
    } 
    while(!q.empty()){
        int k=q.front();
        q.pop();
        for(int i=h[k]; i; i=ed[i].next){
            int a=ed[i].to;
            d[a]=max(d[a], d[k]+v[a]);
            in[a]--;
            if(in[a]==0) q.push(a);
        }
    }
    int ans=0;
    for(int i=1; i<=n; i++) ans=max(ans,d[i]);
    return ans;
}
int main(){
    cin>>n>>m; 
    for(int i=1; i<=n; i++){
        cin>>v[i];
    }
    for(int i=1; i<=m; i++){
        int u, v;
        cin>>u>>v;
        add(u, v);
    }
    for(int i=1; i<=n; i++){
        if(!dfn[i]) tarjan(i);
    }
    for(int i=1; i<=m; i++){
        int x=sd[edge[i].from],y=sd[edge[i].to];
        if(x!=y){
            ed[++s].next=h[x];
            ed[s].to=y;
            ed[s].from=x;
            h[x]=s;
            in[y]++;
        }
    }
    cout<<toposort()<<endl;
    return 0;
}