70分蒟蒻求大佬帮助

#include<bits/stdc++.h>
#define N 500005
using namespace std;
int n,ans=0;
bool vis[N];
vector<int> g[N];
int indegree[N];
queue<int> q;
void topo() {
    for(int i=1;i<=n;i++){
        if(indegree[i]==0){
            q.push(i);
        }
    }
    while(!q.empty()){
        int now=q.front();
        ans++;
        q.pop();
        for(int i=0;i<g[now].size();i++){
            int v=g[now][i];
            indegree[v]--;
            if(!vis[v]) continue;
            if(indegree[v]==0){
                q.push(v);
            }
        }
    }
}
int main(){
    scanf("%d",&n);
    memset(vis,0,sizeof(vis));
    for(int i=1,x,m;i<=n;i++){
        scanf("%d%d",&x,&m);
        vis[x]=1;
        for(int j=1,y;j<=m;j++){
            scanf("%d",&y);
            g[x].push_back(y);
            indegree[y]++;
        }
    }
    topo();
    if(ans==n) printf("YES\n");
    else printf("%d",n-ans);
}