CSES – Round Trip II

#include<bits/stdc++.h>
#define int long long
#define pii pair<int,int>
#define f first
#define s second
#define fastio ios_base::sync_with_stdio(0);cin.tie(0)
using namespace std;
//declare
const int maxn = 1e5+5;
int n,m;
vector<int> G[maxn], v;
int vis[maxn];
//
void dfs(int cur){
    if(vis[cur] == 1){
        for(int i=0;i<v.size();i++){
            if(v[i] == cur){
                cout<<v.size()-i+1<<"\n";
                for(int j=i;j<v.size();j++){
                    cout<<v[j]<<" ";
                }
                cout<<cur<<"\n";
                exit(0);
            }
        }
        return;
    }
    else if(vis[cur] == 2) return;
    v.push_back(cur);
    vis[cur] = 1;
    for(int adj: G[cur]){
        dfs(adj);
    }
    vis[cur] = 2;
    v.pop_back();
}
signed main(){
    fastio;
    cin>>n>>m;
    for(int i=0;i<m;i++){
        int a,b; cin>>a>>b;
        if(a == b) continue;
        G[a].push_back(b);
    }
    for(int i=1;i<=n;i++) if(!vis[i]) dfs(i);
    cout<<"IMPOSSIBLE\n";
    return 0;
}