roundtrip2.cpp

#include <bits/stdc++.h>
//Author : ritchie_s
#define for0(i, n) for (int i = 0; i < (int)(n); ++i) // 0 based indexing
#define for1(i, n) for (int i = 1; i <= (int)(n); ++i) // 1 based indexing
// Short hand for usual tokens
#define pb push_back
#define fi first
#define se second
#define all(x) x.begin(), x.end()
#define sp " "
// Shorthand for commonly used types
using namespace std;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int, int> ii;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<ll> vll;
typedef vector<vll> vvll;
typedef double ld;
// functions
#define MOD 1000000007
ll min(ll a,int b) { if (a<b) return a; return b; }
ll min(int a,ll b) { if (a<b) return a; return b; }
ll max(ll a,int b) { if (a>b) return a; return b; }
ll max(int a,ll b) { if (a>b) return a; return b; }
ll gcd(ll a,ll b) { if (b==0) return a; return gcd(b, a%b); }
ll lcm(ll a,ll b) { return (a/gcd(a,b))*b; }
string to_upper(string a) { for (int i=0;i<(int)a.size();++i) if (a[i]>='a' && a[i]<='z') a[i]-='a'-'A'; return a; }
string to_lower(string a) { for (int i=0;i<(int)a.size();++i) if (a[i]>='A' && a[i]<='Z') a[i]+='a'-'A'; return a; }
bool prime(ll a) { if (a==1) return 0; for (int i=2;i<=round(sqrt(a));++i) if (a%i==0) return 0; return 1; }
// int gcd = __gcd(a,b) , ll LCM = 1LL*a*b/gcd
//solver fn

bool dfs(ll i , map<ll , vll> &adj , vll &vis , vll &parent , ll &start , ll &end){
    vis[i] = 1;
    for(auto x : adj[i]){
        if(vis[x] == 0){
            parent[x] = i;
            if(dfs(x , adj , vis , parent , start , end) == true){
                return true;
            }
        }
        //cycle found
        else if(vis[x] == 1){
            start = x;
            end = i;
            return true;
        }
    }
    vis[i] = 2;
    return false;
}

void solve(){
    ll n,m;
    cin>>n>>m;
    map<ll , vll> adj;
    for0(i,m){
        ll u,v;
        cin>>u>>v;
        adj[u].pb(v);
    }

    vll vis(n+1) , parent(n+1 , -1);
    ll start = -1 , end = -1;
    for(int i = 1 ; i<=n ; i++){
        if(vis[i] == 0){
            if(dfs(i , adj, vis , parent , start , end) == true){
                break;
            }
        }
    }

    if(start == -1){
        cout<<"IMPOSSIBLE"<<endl;
        return;
    }
    else{
        vll cycle;
        cycle.pb(start);
        for(int curr = end ; curr != start ; curr = parent[curr]){
            cycle.pb(curr);
        }
        cycle.pb(start);
        reverse(all(cycle));
        cout<<cycle.size()<<endl;
        for(auto x : cycle){
            cout<<x<<sp;
        }
    }
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.precision(10);
    cout << fixed;
     int t = 1;
  
     while(t--){
        solve();
     }
    return 0;
}