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input/code.cpp: In function 'int main()':
input/code.cpp:63:23: warning: comparison of integer expressions of different signedness: 'std::vector<int>::size_type' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
   63 |     if (result.size() == n) {
      |         ~~~~~~~~~~~~~~^~~~

#include <iostream>
#include <vector>
#include <queue>
#include <set>

using namespace std;

int main() {

    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n, m;
    cin >> n >> m;
    vector<set<int>> adj(n + 1);
    vector<int> in_degree(n + 1, 0);
    bool has_self_loop = false;

    for (int i = 0; i < m; ++i) {
        int a, b;
        cin >> a >> b;

        if (a == b) {
            has_self_loop = true;
            continue; 
        }

        if (adj[a].insert(b).second) {
            in_degree[b]++;
        }
    }

    if (has_self_loop) {
        cout << "IMPOSSIBLE" << endl;
        return 0;
    }

    queue<int> q;
    vector<bool> visited(n + 1, false);

    for (int i = 1; i <= n; ++i) {
        if (in_degree[i] == 0) {
            q.push(i);
            visited[i] = true;
        }
    }

    vector<int> result;

    while (!q.empty()) {
        int u = q.front();
        q.pop();
        result.push_back(u);
        for (int v : adj[u]) {
            in_degree[v]--;
            if (in_degree[v] == 0 && !visited[v]) {
                q.push(v);
                visited[v] = true;
            }
        }
    }

    if (result.size() == n) {
        for (int i = 0; i < n; ++i) {
            cout << result[i] << " \n"[i == n - 1];
        }
    } else {
        cout << "IMPOSSIBLE" << endl;
    }

    return 0;
}

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