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input/code.cpp: In function 'int main()':
input/code.cpp:50:21: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   50 |   for (int i = 1; i < state.size(); i++) {
      |                   ~~^~~~~~~~~~~~~~

#include <bits/stdc++.h>
#include <climits>
#include <vector>

#define ll long long
using namespace std;

void visit(int n, vector<vector<int>> &adj, vector<int> &state,
           vector<int> &res) {
  if (state[n] == 2) {
    return;
  }
  if (state[n] == 1) {
    cout << "IMPOSSIBLE";
    exit(0);
  }
  state[n] = 1;

  for (auto n2 : adj[n]) {
    visit(n2, adj, state, res);
  }

  state[n] = 2;
  res.push_back(n);
}

int main() {
#ifdef LOCAL
  auto in = ifstream("input.txt");
  cin.rdbuf(in.rdbuf());
#endif

  int n, m;
  cin >> n >> m;

  auto adj = vector<vector<int>>(n + 1);

  auto res = vector<int>();
  auto tops = vector<bool>(n + 1, true);

  for (int i = 0; i < m; i++) {
    int a, b;
    cin >> a >> b;
    adj[a].push_back(b);
    tops[b] = false;
  }

  vector<int> state(n + 1, 0);

  for (int i = 1; i < state.size(); i++) {
    if (state[i] == 0)
      visit(i, adj, state, res);
  }

  for (int i = res.size() - 1; i >= 0; i--) {
    cout << res[i] << " ";
  }
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED