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input/code.cpp: In function 'void visit(int, const std::vector<std::vector<int> >&, std::vector<int>&, std::__cxx11::list<int>&)':
input/code.cpp:21:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   21 |     for (int i = 0; i < adj[node].size(); i++)
      |                     ~~^~~~~~~~~~~~~~~~~~

#include <iostream>
#include <vector>
#include <list>
#include <algorithm>

using namespace std;

void visit(int node, const vector<vector<int>>& adj, vector<int> &vis, list<int>& result)
{
    if (vis[node] == 2)
    {
        return;
    }
    if (vis[node] == 1)
    {
        cout << "IMPOSSIBLE" << endl;
        exit(0);
    }

    vis[node] = 1;
    for (int i = 0; i < adj[node].size(); i++)
    {
        visit(adj[node][i], adj, vis, result);
    }
    vis[node] = 2;
    result.push_front(node);
}
int main()
{
    int n, m;
    cin >> n >> m;
    vector<vector<int>> adj(n+1);
    for (int i = 0; i < m; i++)
    {
        int a, b;
        cin >> a >> b;
        adj[a].push_back(b);
    }
    vector<int> vis(n+1, 0);
    list<int> result;
    for (int i = 1; i <= n; i++)
    {
        if (vis[i] == 0)
        {
            visit(i, adj, vis, result);
        }
    }
    for (int node : result)
    {
        cout << node << " ";
    }
}

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