#include <iostream>
#include <string>
#define Node node *

typedef struct node
{
    int data;
    Node pred;
    Node succ;
} node;

class List
{
private:
    Node header = new node;
    Node trailer = new node;
    int _size;

protected:
    Node firstInsertN(Node New);
    Node lastInsertN(Node New);
    Node succInsertN(Node old, Node ne);
    Node predInsertN(Node old, Node ne);
    void removeN(Node i);
    Node searchN(int value,int n,Node p);

public:
    int getSize()
    {
        return _size;
    }
    List();
    ~List();
    void clear();
    Node first() { return header->succ; }
    Node last() { return trailer->pred; }
    int firstInsert(int New);
    int lastInsert(int New);
    Node succInsert(Node Old, int New);
    Node predInsert(Node Old, int New);

    int get(int i);
    Node find(int i);
    int remove(int i);
    Node searchN(int value,int n);
    int valid(Node p);
    

    void sort();

    int operator[](int i)
    {
        Node p = first();
        while (0 < i--)
            p = p->succ;
        return p->data;
    }
};