拓扑排序

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
ll edge[510], a[510];
vector<ll> ve[510];
ll n, ans;
bool v[510];
queue<ll> q;
int main()
{
    cin >> n;
    for (int i = 1; i <= n; i++) {
        ll m, y;
        cin >> a[i];
        cin >> m;
        v[a[i]] = 1; // 记录a[i]处有摄像头
        for (int j = 1; j <= m; j++) {
            cin >> y;
            if(a[i]==y) continue;//避免自环
            ve[a[i]].push_back(y);
            edge[y]++;
        }
    }
    for (int i = 1; i <= n; i++) {
        if (!edge[a[i]])
            q.push(a[i]); // 加入入度为0节点
    }
    while (!q.empty()) {
        ans++; // 计数器
        ll x = q.front();
        q.pop(); // 取出队首
        for (int i = 0; i < ve[x].size(); i++) {
            ll y = ve[x][i];
            edge[y]--; // 入度减一
            if (!edge[y] && v[y])
                q.push(y); // 如果这个地方有摄像头且入度为0
        }
    }
    if (ans == n)
        printf("YES\n");
    else
        printf("%lld\n", n - ans);
    return 0;
}