Course Schedule

#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <climits>

using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    // freopen("input.txt", "r", stdin); // TODO: REMOVE THIS YOU STUPID ****

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

    vector<vector<int> > g(n + 1);
    for (int i = 0; i < m; i++) {
        int u, v;
        cin >> u >> v;
        g[u].push_back(v);
    }

    vector<int> color(n + 1, 0); // 0=unvisited, 1=visiting, 2=done
    vector<int> topo;
    bool hasСycle = false;

    for (int s = 1; s <= n; s++)
        if (color[s] == 0) {
            vector<pair<int, int> > st; // (node, next_child_idx)
            st.push_back({s, 0});
            color[s] = 1;

            while (!st.empty() && !hasСycle) {
                auto &[u, i] = st.back();

                if (i < (int) g[u].size()) {
                    int v = g[u][i++];
                    if (color[v] == 0) {
                        color[v] = 1;
                        st.push_back({v, 0});
                    } else if (color[v] == 1) {
                        hasСycle = true; // back edge
                    }
                } else {
                    color[u] = 2;
                    topo.push_back(u);
                    st.pop_back();
                }
            }
            if (hasСycle) break;
        }

    if (hasСycle) {
        cout << "IMPOSSIBLE\n";
        return 0;
    }

    reverse(topo.begin(), topo.end());
    for (int x: topo) cout << x << " ";
    cout << "\n";
}

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