[Round Trip II](https://cses.fi/problemset/task/1678/)

[ShenNaizhi]

Hello Ankit Priyarup, I have read and understood your code for problem site, but I encountered an issue while reproducing it:

I made some modifications to your code and got an AC. Here is my submission: AC code
I also submitted your code: WA code
Hope that I didn't make a mistake with your code.

It seems that VJudge only allows users to check the submissions in the form of pictures. For convenience, I provide my code here:

// #undef LOCAL
// #undef LOCAL_IO

#include <bits/stdc++.h>
using i64 = long long;
using namespace std;

#ifdef LOCAL
#   define debug(x) cout << #x << " = " << x << endl
#   define debugv(v) do { cout << #v << ": ["; string delim = "";\
    for (const auto& e : v) cout << delim << e, delim = ", ";\
    cout << ']' << endl; } while (0)
#else
#   define debug(...) 12
#   define debugv(...) 12
#endif

// returns the source of the cycle, otherwise -1
int dfs(vector<vector<int>>& g, vector<bool>& vis, vector<bool>& unsol,
        int cur, vector<int>& ans, bool& found) {
    vis[cur] = true;
    int res = -1;
    for (int adj : g[cur]) {
        if (!vis[adj]) {
            res = dfs(g, vis, unsol, adj, ans, found);
        } else if (!unsol[adj]) {
            // the problem has guaranteed no loops
            ans = {adj, cur};
            return adj;
        }
        if (res != -1) {
            if (!found) ans.push_back(cur);
            if (cur == res) {
                found = true;
                reverse(ans.begin(), ans.end());
            }
            return res;
        }
    }
    unsol[cur] = true;
    return -1;
}

void solve() {
    int n, m;
    cin >> n >> m;
    vector<vector<int>> g(n + 1); // vertex: [1..n]
    for (int i = 0; i < m; i++) {
        int a, b;
        cin >> a >> b;
        g[a].push_back(b);
    }
    vector<bool> vis(n + 1), unsol(n + 1);
    vector<int> ans;
    bool found = false;
    for (int i = 1; i <= n; i++) {
        int succ = -1;
        if (!vis[i]) dfs(g, vis, unsol, i, ans, found);
        if (found) break;
        else ans.clear();
    }
    if (!found) {
        cout << "IMPOSSIBLE" << "\n";
    } else {
        cout << ans.size() << "\n";
        for (int i = 0; i < ans.size(); i++) cout << ans[i] << " \n"[i == ans.size() - 1];
    }
}

int main() {
    #ifdef LOCAL_IO
        freopen("t.in", "r", stdin);
        freopen("t.out", "w", stdout);
    #endif
    cin.tie(0);
    ios::sync_with_stdio(0);
    int _ = 1;
    // cin >> _;

    while (_--) {
        solve();
    }
    return 0;
}

Thank you again for your solution code!