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);
    dfs(graph,node);
  }
  solution.emplace_back(u);  
}
#define IM puts("IMPOSSIBLE");
int main(){
  int n,m,a,b;
  cin>>n>>m;
  set<int>graph[n+1];
  vector<int>indegree(n+1,0),outdegree(n+1,0);
  for(int i=0;i<m;i++){
    cin>>a>>b;
    graph[a].insert(b);
    indegree[b]++;
    outdegree[a]++;
  }
if(indegree[1]!=outdegree[1]-1){
  IM
  return 0;
}
 
for(int i=2;i<n;i++){
if(indegree[i]!=outdegree[i]){
  IM
  return 0;
}
}
if(indegree[n]-1!=outdegree[n]){
  IM
  return 0;
}
  dfs(graph,1);
if(solution.size()!=m+1){
  IM
  return 0;
}  
  reverse(solution.begin(),solution.end());
  for(int i:solution)cout<<i<<" ";
  return 0;
}