Shared code

#include<iostream>
#include<algorithm>
#include<vector>
#include<set>
using namespace std;
vector<int>solution;
int node;
void dfs(set<int>*graph,int u){
  while(!graph[u].empty()){
    node=*graph[u].begin();
    graph[u].erase(node);
    graph[node].erase(u);   
    dfs(graph,node);
  }
  solution.emplace_back(u);
}
inline void im(){
    cout<<"IMPOSSIBLE";
}
int main(){
  int n,m,a,b,cntEven=0,len;
  cin>>n>>m;
  len=m;
  vector<int>deg(n+1,0);
  set<int>graph[n+1];
  while(m--){
    cin>>a>>b;
    deg[a]++;
    deg[b]++;
    graph[a].insert(b);
    graph[b].insert(a);
  }
  int start=1;
  for(int i=1;i<=n;i++){
    cntEven+=deg[i]%2==0;
    if(deg[i]&1){
      start=i;
    }
  }

  if(cntEven!=n and cntEven!=n-2){
    im();
    return 0;
  }
  dfs(graph,start);
  if(solution.size()!=(len+1)){
    im();
    return 0;
  }
  if(solution[0]!=1 or solution.back()!=1){
    im();
    return 0;
  }
 for(int i:solution)cout<<i<<" ";
  return 0;
}