缩点（模板）

https://www.luogu.com.cn/problem/P3387

#include<bits/stdc++.h>
#define lc p<<1
#define rc p<<1|1
#define INF 1e18
using namespace std;
#define lowbit(x) x&(-x)
#define endl '\n'
using ll = long long;
using int128 = __int128;
using pii = pair<ll,ll>;
const double PI = acos(-1);
const int N=1e4+10;
int dfn[N],low[N],tot;
int stk[N],top,dp[N],sum[N];
int scc[N],num;
int a[N];
bool instk[N];
vector<int> e[N],nowe[N];
//记忆化搜索
int dfs(int u){
	if(dp[u]!=-1) return dp[u];
	dp[u]=sum[u];
	for(auto v:nowe[u]){
		dp[u]=max(dp[u],sum[u]+dp[v]);
	}
	return dp[u];
}
void tarjan(int u){
	dfn[u]=low[u]=++tot;
	stk[++top]=u;
	instk[u]=1;
	for(auto v:e[u]){
		if(!dfn[v]){
			tarjan(v);
			low[u]=min(low[u],low[v]);
		}
		else if(instk[v]){
			low[u]=min(low[u],dfn[v]);
		}
	}
	if(low[u]==dfn[u]){
		int p=-1;
		num++;
		do{
			p=stk[top--];
			instk[p]=0;
			scc[p]=num;
			sum[num]+=a[p];
		}while(p!=u);
	}
}
void solve(){
	memset(dp,-1,sizeof(dp));
	int n,m;cin>>n>>m;
	for(int i=1;i<=n;i++){
		cin>>a[i];
	}
	while(m--){
		int u,v;cin>>u>>v;
		e[u].push_back(v);
	}
	for(int i=1;i<=n;i++){
		if(!dfn[i]) tarjan(i);
	}
	for(int i=1;i<=n;i++){
		for(auto j:e[i]){
			if(scc[j]!=scc[i]){
				nowe[scc[i]].push_back(scc[j]);
			}
		}
	}
	int ans=-1;
	for(int i=1;i<=num;i++){
		ans=max(ans,dfs(i));
	}
	cout<<ans;
}


int main() {
//	ios::sync_with_stdio(false);
//	cin.tie(nullptr), cout.tie(nullptr);
	
	int T = 1;
//	cin>>T;
	while (T--) {
		solve();
	}
	
	return 0;
}