Shared code

#include<bits/stdc++.h>
using namespace std;
int main() {
// #ifndef ONLINE_JUDGE
//     freopen("input.txt", "r", stdin);
//     freopen("output.txt", "w", stdout);
// #endif

    int n, m;
    cin >> n >> m;
    int id[n];
    memset(id, 0, sizeof(id));
    vector<vector<int>> adj(n);
    for (int i = 0; i < m; i++) {
        int x, y;
        cin >> x >> y;
        adj[x - 1].push_back(y - 1);
        id[y - 1]++;
    }
    queue<int> q;
    for (int i = 0; i < n; i++) {
        if (id[i] == 0)
            q.push(i);
    }
    vector<int> ans;
    while (!q.empty()) {
        ans.push_back(q.front());
        for (int i = 0; i < adj[q.front()].size(); i++) {
            id[adj[q.front()][i]]--;
            if (id[adj[q.front()][i]] == 0) {
                q.push(adj[q.front()][i]);
            }
        }
        q.pop();
    }
    int sp[n];
    sp[0] = 0;
    for (int i = 1; i < n; i++)
        sp[i] = INT_MAX;
    int p[n];
    for (int i = 0; i < n; i++)
        p[i] = i;
    int ind;
    for (int i = 0; i < ans.size(); i++) {
        if (ans[i] == 0) {
            ind = i;
            break;
        }
    }
    for (int i = ind; i < ans.size(); i++) {
        for (int j = 0; j < adj[ans[i]].size(); j++) {
            if (sp[ans[i]] - 1 < sp[adj[ans[i]][j]]) {
                sp[adj[ans[i]][j]] = sp[ans[i]];
                p[adj[ans[i]][j]] = ans[i];
            }

        }
    }
    vector<int> a;
    int i = n - 1;
    while (p[i] != i && i != 0) {
        a.push_back(i);
        i = p[i];
    }
    a.push_back(i);
    if ((*(a.end() - 1)) != 0)
        cout << "IMPOSSIBLE" << endl;
    else {
        reverse(a.begin(), a.end());
        cout << a.size() << endl;
        for (int x : a)
            cout << x + 1 << " ";
        cout << endl;
    }

    return 0;
}