#include <iostream>
#include <list>

using namespace std;

class Deque {
  int temp;
  list<int> l;
  public:
    void push(int);
    int pop();
    void inject(int);
    int eject();
};

void Deque::push(int a){
  l.push_front(a);
}

int Deque::pop(){
  if (l.size() == 0){
    return 0;
  }
  temp = l.front();
  l.pop_front();
  return temp;
}

void Deque::inject(int a){
  l.push_back(a);
}

int Deque::eject(){
  if (l.size() == 0){
    return 0;
  }
  temp = l.back();
  l.pop_back();
  return temp;
}

int main(){
  Deque d;
  d.inject(4);
  cout << d.eject() << endl;
  cout << d.eject() << endl;

  d.push(12);
  cout << d.pop() << endl;
  cout << d.eject() << endl;

  return 0;
}