RoundTrip.cpp

#include<bits/stdc++.h>
using namespace std;

vector<vector<int>>adj(100001);
vector<int>ans(100000);


int dfs(int x,int i,int indice,vector<bool>&check){
    if(indice && x == i){return indice;}
    check[x] = 0;

    int a = 0;

    for(int z : adj[x]){
        ans[indice] = z;
        a = a || dfs(z,i,indice+1,check);
        if(a)break;
    }

    return a;
}




int main(){
    int n,m;
    cin >> n >> m;

    while(m--){
        int x,y;
        cin >> x >> y;
        adj[x].push_back(y);
        adj[y].push_back(x);
    }

    int something = 0;
    vector<bool>check(n+1,false);
    for(int i = 1 ; i <= n ; i ++){
        if(check[i])continue;
        something = dfs(i,i,0,check);
        if(something)break;
    }

    for(int i = 0 ; i < something ; i ++)cout << ans[i];

    return 0;
}