Shared code

#include <bits/stdc++.h>
#define uwu return 0;

using namespace std;

const int SIZE = 2e5 + 5;

vector <pair<int, int>> graph[SIZE];

vector <int> path;

#define fs first
#define sc second

bool been[SIZE];

bool check_able(int V){
    for (int i = 1; i <= V; i++){
        if(graph[i].size() & 1)
            return 0;
    }
    return 1;
}

int cur[SIZE];

void dfs(int nd){
    for (; cur[nd] < graph[nd].size(); cur[nd]++){
        if(!been[graph[nd][cur[nd]].sc]){
            been[graph[nd][cur[nd]].sc] = 1;
            dfs(graph[nd][cur[nd]].fs);
        }
    }
    path.push_back(nd);
    return;
}

int main(){
    cin.tie(0), ios::sync_with_stdio(0);
    int V, E;
    cin >> V >> E;
    for (int u, v, i = 0; i < E; i++){
        cin >> u >> v;
        graph[u].push_back({v, i});
        graph[v].push_back({u, i});
    }

    // check the amount of odd nodes
    if(!check_able(V)){
        cout << "IMPOSSIBLE\n";
        uwu
    }

    // find cycle
    dfs(1);
    
    // check connectivity
    if(path.size() != E + 1){
        cout << "IMPOSSIBLE\n";
        uwu
    }

    for(auto i:path){
        cout << i << ' ';
    }
    cout << '\n';
    uwu
}