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];

int in_deg[SIZE], out_deg[SIZE];

bool check_able(int V, int &out_nd){
    int in_nd;
    bool flag_found_in = 0, flag_found_out = 0;
    for (int i = 1; i <= V; i++){
        if(abs(out_deg[i] - in_deg[i]) >= 2)
            return 0;
        if(out_deg[i] - in_deg[i] == 1){
            if(flag_found_out)
                return 0;
            flag_found_out = 1;
            out_nd = i;
        }
        if(in_deg[i] - out_deg[i] == 1){
            if(flag_found_in)
                return 0;
            flag_found_in = 1;
            in_nd = i;
        }
    }
    return (out_nd == 1 && in_nd == V);
}

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});
        in_deg[v]++;
        out_deg[u]++;
    }

    // check the amount of odd nodes
    int rt = 0;
    if(!check_able(V, rt)){
        cout << "IMPOSSIBLE\n";
        uwu
    }

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

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