CSES – Longest Flight Route

#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,in[maxn],dis[maxn],p[maxn];
vector<int> G[maxn], ord;
bool vis[maxn], flag;
//
void topo_sort(){
    queue<int> q;
    for(int i=1;i<=n;i++) if(!in[i]) q.push(i);
    while(q.size()){
        auto cur = q.front(); q.pop();
        ord.push_back(cur);
        for(int adj: G[cur]){
            if(!--in[adj]) q.push(adj);
        }
    }
}
void dfs(int cur){
    if(vis[cur]) return;
    vis[cur] = 1;
    flag |= (cur == n);
    for(int adj: G[cur]){
        dfs(adj);
    }
}
signed main(){
    fastio;
    cin>>n>>m;
    for(int i=0;i<m;i++){
        int a,b; cin>>a>>b;
        G[a].push_back(b);
        in[b]++;
    }
    dfs(1);
    if(!flag) return cout<<"IMPOSSIBLE\n", 0;
    topo_sort();
    for(int i=1;i<=n;i++) dis[i] = -1e18;
    dis[ord[0]] = 0;
    for(int cur: ord){
        for(int adj: G[cur]){
            if(dis[adj] < dis[cur] + 1){
                p[adj] = cur;
                dis[adj] = max(dis[adj], dis[cur] + 1);
            }
        }
    }
    int now = n;
    vector<int> ans;
    while(now != 1){
        ans.push_back(now);
        now = p[now];
    }
    reverse(ans.begin(),ans.end());
    cout<<ans.size()+1<<"\n"<<1<<" ";
    for(int i:ans) cout<<i<<" ";
    cout<<"\n";
    return 0;
}