Shared code

#include<bits/stdc++.h>
using namespace std;
class RoundTrip{
  int n,m;
  vector<int> *g;
  int start,end;
  public:
      vector<int>p;
    RoundTrip(int n,int m,vector<int> g[]){
      this->n=n;
      this->m=m;
      this->g=g;
      p.resize(n+1,0);
      start=end=-1;
    }
    bool dfs(int u,int parent){
        for(int v:g[u]){
          if(v!=parent){
            if(p[v]==0){
              p[v]=u;
              if(dfs(v,u))return true;
            }else{
              start=u;
              end=v;
              return true;
            }
          }
        } 
        return false;      
    }
  void printLoop(){
    if(start==-1){
      cout<<"IMPOSSIBLE\n";
      return;
    }
     vector<int>ans(1,end);
      while(start!=end){
        ans.push_back(start);
        start=p[start];
        }
      ans.push_back(start);
      cout<<ans.size()<<endl;
      for(auto i:ans)cout<<i<<" ";
  }
};
int main(){
  int n,m,a,b;
  cin>>n>>m;
  vector<int>g[n+1];
  for(int i=0;i<m;i++){
    cin>>a>>b;
    g[a].push_back(b);
    g[b].push_back(a);
  }
  RoundTrip ans(n,m,g);
  for(int i=n;i>=1;i--){
    if(ans.p[i])continue;
    if(ans.dfs(i,-1))break;
  }
 ans.printLoop();
  return 0;
}