#include <iostream>

/*
    <prev-- item --next> <prev-- _first --next> <prev-- second element --next>
   <prev-- ...
*/

class List {
 public:
  struct Item {
    int value{};
    Item* previous{};
    Item* next{};
  };

  List() = default;
  List(List const& second_list) {}
  ~List() {
    for (unsigned i = 0; i < size(); i++) {
      delete_front();
    }
  }

  void push_front(int value) {
    Item* item = new Item{};
    item->value = value;

    if (is_empty()) {
      _first = item;
      _last = item;
    } else {
      item->next = _first;
      _first->previous = item;
      _first = item;
    }

    _size++;
  }

  void push_back(int value) {}

  int pop_front() {
    if (is_empty()) {
      return 0;
    }
    int first_value = _first->value;
    delete_front();
    return first_value;
  }

  int pop_back() {}

  bool is_empty() const { return size() == 0; }
  unsigned size() const { return _size; }

 private:
  void delete_front() {
    Item* second_element = _first->next;
    delete _first;
    _first = second_element;
    _size--;
  }

  Item* _first{};
  Item* _last{};
  unsigned _size{};
};

int main() {}