不考虑优先限制的

#include <bits/stdc++.h>
using namespace std;
int n, m;
vector<int> e[100000 + 5];
int d[100000 + 5];
queue<int> q;
vector<int> ans;
int main()
{
    int t;
    cin >> t;
    while (t--)
    {
        cin >> n >> m;
        //-----清空 start-----
        for (int i = 1; i <= n; i++)
        {
            e[i].clear();
            d[i] = 0;
        }
        ans.clear();
        while (!q.empty())
            q.pop();
        //-----清空 end-----
        for (int i = 1; i <= m; i++)
        {
            int u, v;
            cin >> u >> v;
            e[u].push_back(v);
            d[v]++;
        }
        for (int i = 1; i <= n; i++)
            if (d[i] == 0)
                q.push(i);
        bool flag = false; // 多解标记
        while (!q.empty())
        {
            if (q.size() > 1)
                flag = true;
            int now = q.front();
            q.pop();
            ans.push_back(now);
            for (int i = 0; i < e[now].size(); i++)
            {
                int v = e[now][i];
                d[v]--;
                if (d[v] == 0)
                    q.push(v);
            }
        }
        if (ans.size() != n)
        {
            cout << "Impossible!\n";
        }
        else
        {
            for (int i = 0; i < ans.size(); i++)
                cout << ans[i] << " ";
            cout << "\n";
        }
    }
    return 0;
}

满分

#include <bits/stdc++.h>
using namespace std;
int n, m;
vector<int> e[100000 + 5];
int d[100000 + 5];
priority_queue<int> q; // ~~~ 反图的大的优
vector<int> ans;
int main()
{
    int t;
    cin >> t;
    while (t--)
    {
        cin >> n >> m;
        //-----清空 start-----
        for (int i = 1; i <= n; i++)
        {
            e[i].clear();
            d[i] = 0;
        }
        ans.clear();
        while (!q.empty())
            q.pop();
        //-----清空 end-----
        for (int i = 1; i <= m; i++)
        {
            int u, v;
            cin >> v >> u; // ~~~ 存反图
            e[u].push_back(v);
            d[v]++;
        }
        for (int i = 1; i <= n; i++)
            if (d[i] == 0)
                q.push(i);
        bool flag = false; // 多解标记
        while (!q.empty())
        {
            if (q.size() > 1)
                flag = true;
            int now = q.top();
            q.pop();
            ans.push_back(now);
            for (int i = 0; i < e[now].size(); i++)
            {
                int v = e[now][i];
                d[v]--;
                if (d[v] == 0)
                    q.push(v);
            }
        }
        if (ans.size() != n)
        {
            cout << "Impossible!\n";
        }
        else
        {
            //~~~ 原本顺序是反向的大的优先，所以要倒着输出
            for (int i = (int)ans.size() - 1; i >= 0; i--)
                cout << ans[i] << " ";
            cout << "\n";
        }
    }
    return 0;
}