Language: C++

#include<bits/stdc++.h>
#include<iostream>
#include<cmath>
#include<vector>
#include<string>
#include<queue>
#include<set>
#include<map>
#include<algorithm>
#include<functional>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef pair<ll, ll> pll;
typedef pair<ull, ull> pullull;
typedef pair<ll, int> plli;
typedef pair<int, pii> pipii;
typedef vector<vector<int> > mati;
typedef vector<vector<double> > matd;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<vector<vector<ll>>> vvvll;
typedef vector<bool> vb;
typedef vector<vector<bool>> vvb;
typedef vector<vector<vector<bool>>> vvvb;

#define FOR(i,x,y) for(ll i=(ll)x; i<(ll)y; ++i)
#define REP(i,y) FOR(i, 0, y)
#define RFOR(i,x,y) for(ll i=(ll)x; i>=(ll)y; --i)
#define RREP(i,x) RFOR(i, x, 0)

template <typename T>
void vec_print(vector<T> VEC){
  REP(i, VEC.size()){
    cout << VEC[i] << " ";
  }
  cout << "\n";
};

template <typename T>
void mat_print(vector<vector<T> > MAT){
  REP(i, MAT.size()){
    REP(j, MAT[i].size()){
      cout << MAT[i][j] << " ";
    }
    cout << "\n";
  }
};

template <typename CLASS1, typename CLASS2>
class HOGE{
  public:
    CLASS1 key;
    CLASS2 value;
    HOGE(void){
      return;
    };
    HOGE(CLASS1 key, CLASS2 value){
      this->key = key;
      this->value = value;
    };
    ~HOGE(void){
      return;
    };

    void print(void){
      cout << "key : " << key << ", value : " << value << "\n";
      return;
    };
    
    bool operator==(const HOGE &obj){
      return (this->value == obj.value);
    };
    bool operator<(const HOGE &obj){
      return (this->value < obj.value);
    };
    bool operator>(const HOGE &obj){
      return (this->value > obj.value);
    };
};

constexpr int INF = (1<<30);
constexpr ll INFLL = 1LL<<62;
constexpr long double EPS = 1e-12;
constexpr ll MOD = (ll)((1E+9)+7);

vvll Path;

void forward_search(deque<ll>& dq, vb& cand, ll& num_ans){
  ll pos = dq.back();
  for(ll i=0; i<Path[pos].size(); ++i){
    if(!cand[Path[pos][i]]) continue;
    dq.push_back(Path[pos][i]);
    num_ans++;
    cand[Path[pos][i]] = false;
    forward_search(dq, cand, num_ans);
    break;
  }
  return;
}

void backward_search(deque<ll>& dq, vb& cand, ll& num_ans){
  ll pos = dq.front();
  for(ll i=0; i<Path[pos].size(); ++i){
    if(!cand[Path[pos][i]]) continue;
    dq.push_front(Path[pos][i]);
    num_ans++;
    cand[Path[pos][i]] = false;
    backward_search(dq, cand, num_ans);
    break;
  }
  return;
}

int main(){
  cin.tie(0); // cut the cin and cout (default, std::flush is performed after std::cin)
  ios::sync_with_stdio(false); // cut the iostream and stdio (DON'T endl; BUT "\n";)

  ll N, M;
  cin >> N >> M;
  Path.resize(N+2);
  ll A, B;
  REP(i, M){
    cin >> A >> B;
    Path[A].push_back(B);
    Path[B].push_back(A);
  }

  vb cand(true, N+2);
  deque<ll> dq;
  ll ans_num = 1;
  dq.push_back(A);
  cand[A] = false;

  forward_search(dq, Path, cand, ans_num);
  backward_search(dq, Path, cand, ans_num);

  printf("%d\n", ans_num);
  printf("%d", dq.front());
  dq.pop_front();
  while(!dq.empty()){
    printf(" %d", dq.front());
    dq.pop_front();
  }
  printf("\n");

  return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:138:22: error: invalid initialization of reference of type 'vb&' {aka 'std::vector<bool>&'} from expression of type 'vvll' {aka 'std::vector<std::vector<long long int> >'}
  138 |   forward_search(dq, Path, cand, ans_num);
      |                      ^~~~
a.cc:92:40: note: in passing argument 2 of 'void forward_search(std::deque<long long int>&, vb&, ll&)'
   92 | void forward_search(deque<ll>& dq, vb& cand, ll& num_ans){
      |                                    ~~~~^~~~
a.cc:139:23: error: invalid initialization of reference of type 'vb&' {aka 'std::vector<bool>&'} from expression of type 'vvll' {aka 'std::vector<std::vector<long long int> >'}
  139 |   backward_search(dq, Path, cand, ans_num);
      |                       ^~~~
a.cc:105:41: note: in passing argument 2 of 'void backward_search(std::deque<long long int>&, vb&, ll&)'
  105 | void backward_search(deque<ll>& dq, vb& cand, ll& num_ans){
      |                                     ~~~~^~~~