Lesson17

#include <iostream>
#include <vector>

using namespace std;

int main() {
  int n = 0; cin >> n;
  int m = 0; cin >> m;
  vector<vector<int>> out_edges(n);
  for (int i = 0; i < m; ++i) {
    int a, b; cin >> a >> b;
    out_edges[a].push_back(b);
  }

  vector<vector<int>> in_edges(n);
  for (int u = 0; u < n; ++u) {
    // for (int k = 0; k < out_edges[i].size(); ++k) {
    //   int j = out_edges[i][k];
    // for (auto k = out_edges[i].begin(); k != out_edges[i].end(); ++k) {
    //   int j = *k;
    for (int v : out_edges[u])
      in_edges[v].push_back(u);
  }

  for (int u = 0; u < n; ++u) {
    cout << u << ": ";
    for (int x : in_edges[u]) cout << x << " ";
    cout << endl;
  }
  return 0;
}