#include <iostream>
#include <iterator>
#include <memory>

using namespace std;

struct Node {
    int data;
    Node * prev;
    Node * next;

    Node() {
    }
};

class List_iterator {
private:
    Node * it;
public:
    List_iterator(Node * input) {
        it = input;
    }
    int operator*() {
        return it->data;
    }
    List_iterator& operator++() {
        it = it->next;
        return *this;
    }
    List_iterator& operator--() {
        it = it->prev;
        return *this;
    }
};

class List {
private:
    Node * first;
    Node * last;
    size_t list_size;
public:
    List() {
        auto fisrt = new Node;
        last = first;
        list_size = 0;
    }
    ~List() {
        /*while (first) {
            last = first->next;
            delete first;
            first = last;
        }*/
    }
    size_t size() {
        return list_size;
    }
    void push_back(int elem) {
        Node * tmp = new Node;
        if (first) {
            std::cout << "wtf";
        }
        /*last->data = elem;
        last->next = tmp;
        tmp->prev = last;
        tmp->next = nullptr;
        last = tmp;
        list_size += 1;*/
    }
    void push_front(int elem) {
        Node * tmp = new Node;
        tmp->data = elem;
        tmp->prev = nullptr;
        if (first != nullptr) {
            tmp->next = first;
            first->prev = tmp;
            first = tmp;
        }
        else {
            tmp->next = last;
            first = tmp;
            last->prev = tmp;
        }
        list_size += 1;
    }
    int pop_back() {
        if (first != nullptr) {
            int result = (last->prev)->data;
            Node * tmp = last->prev->prev;
            tmp->next = last;
            delete last->prev;
            last->prev = tmp;
            list_size -= 1;
            return result;
        }
    }
    int pop_front() {
        if (first != nullptr) {
            int result = first->data;
            Node * tmp = first->next;
            tmp->prev = nullptr;
            delete first;
            first = tmp;
            return result;
            list_size -= 1;
        }
    }
    List_iterator begin() {
        return List_iterator(first);
    }
    List_iterator end() {
        return List_iterator(last);
    }
};