#include <bits/stdc++.h>   using namespace std;   int main() {     ios_base::sync_with_stdio(false);     cin.tie(NULL);     int t;     cin >> t;     while (t--) {       int n, m;       cin >> n >> m;       vector<int> g[n];       for (int i = 0; i < m; i++) {         int u, v;         cin >> u >> v;         u--, v--;         g[v].push_back(u);       }       int l[n];       for (int i = 0; i < n; i++) l[i] = 0;       vector<int> a;       for (int i = 0; i < n; i++) {         for (auto j : g[i]) l[i] = max(l[i], l[j] + 1);         if (l[i] == 2) {           l[i] = -1;           a.push_back(i);         }       }       cout << a.size() <<   n ;       for (auto i : a) cout << i + 1 <<    ;       cout <<   n ;     }   }   