#include <iostream>
using namespace std;

template <typename T> class list
{
    struct element
    {
        T data;
        element * prev;
        element(T x, element *elem): data(x), prev(elem){};
    };
    int length;
    element* top;
    public:

    list ()
    {
        length = 0;
        top = NULL;
    }
    bool empty()
    {
        if (length == 0)
            return 1;
        else return 0;
    }
    void push_front (T x)
    {
        insert(x, 0);
    }
    void pop_front ()
    {
        T templength = length;
        element *temptop = top;
        if (length > 0)
        {
            while (templength != 2)
            {
                temptop = temptop->prev;
                templength--;
            }
        }
        temptop->prev = NULL;
        delete temptop;
        length--;
    }
    void insert (T i, T x)
    {
        if (i + 1 == length)
            push_back(x);
        if ((length >= i + 1) && i >= 0)
        {
            T templength = length;
            element* temp = new element (top->data, top);
            top = temp;
            temp = top;
            temp = temp->prev;
            while (templength != i + 1)
            {
                temp->data = temp->prev->data;
                temp = temp->prev;
                templength--;
            }
            temp->data = x;
            delete temp;
            length++;
        }
    }
    T back()
    {
        return top->data;
    }
    T front()
    {
        T templength = length;
        element *temptop = top;
        if (length > 0)
        {
            while (templength != 1)
            {
                temptop = temptop->prev;
                templength--;
            }
        }
        T res = temptop->data;
        delete temptop;
        return res;
    }
    void pop_back()
    {
        top = top->prev;
        length --;
    }

    void push_back (T x)
    {
        element* temp = new element (x, top);
        top = temp;
        length ++;
        delete temp;
    }
    void clear()
    {
        top = 0;
        length = 0;
    }
    void resize( int i, T value)
    {
        if (i > length)
        {
            while (i != length)
            {
                push_back(value);
            }
        }
    }
    void resize( int i)
    {
        while (length != i)
        {
            pop_back();
        }
    }
    int size ()
    {
        return length;
    }
    void remove(T value)
    {
        T templength = length;
        element *temptop  = top;
        int k = 0;
        while (templength != 1)
        {
            if ( k == 1)
            {
                if (temptop->prev->data == value)
                {
                    temptop->prev = temptop->prev->prev;
                    length--;
                }
                else
                    temptop = temptop->prev;
                templength--;
            } else {
                if (top->data == value)
                {
                    top = top->prev;
                    temptop = top;
                    length--;
                    templength--;
                }
                k = 1;
            }

        }
     }
    T& operator[](int i)
    {
            T templength = length;
            element* temptop = top;
            while (templength != i + 1)
            {
                temptop = temptop->prev;
                templength--;
            }
            T res = temptop->data;
            delete temptop;
            return res;
    }
    void swap(element *f, element *s)
    {
        int check = 0;
        if (s -> prev == top)
            check = 1;
        if (s -> prev == f)
        {
            s ->prev = f ->prev ;
            f ->prev = s ->prev ->prev;
            s ->prev ->prev = f;
        } else
        {
            element *fdop = f ->prev;
            element *sdop = s ->prev;
            element *prfdop = fdop -> prev;
            s ->prev = fdop;
            fdop ->prev = sdop ->prev;
            f ->prev = sdop;
            sdop ->prev = prfdop;
        }
        if (check == 1)
                top = s->prev;
    }
    void sort()
    {
        for (int k = 1; k <= length; k++)
        {
            element* temptop = new element(NULL, top);
            for (int templength = length; templength > k; templength--)
            {
                if (temptop->prev ->prev->data > temptop ->prev ->data)
                {
                    swap(temptop->prev ,temptop);
                }
                temptop = temptop-> prev;
            }
            delete temptop;
        }
    }
    element *end()
    {
        element* temptop = new element(NULL, top);
        top = temptop;
        temptop = top;
        delete temptop;
        return top->prev;
    }
    element *begin()
    {
        element* temptop = top;
        T templength = length;
        for (; templength > 0; templength--)
        {
            temptop = temptop->prev;
        }
        return temptop-> prev;
    }
    struct Iterator
    {
        public:
        element *it;
        void operator= (element *out)
        {
            it = out ;
        }
        T operator* ()
        {
            return it->data;
        }
        element* operator--()
        {
            it = it->prev;
            return it;
        }
        bool operator == (element* temp)
        {
            if (it == temp)
                return 1;
            return 0;
        }
        bool operator!= (element* temp)
        {
            if (it != temp)
                return 1;
            return 0;
        }
    };
    ~list ()
    {
        delete top;
    }
};


int main()
{
    list<int> a;
    a.push_back(9);
    a.push_back(5);
    a.push_back(1);
    a.push_back(4);
    for (int i = 0; i < a.size(); i ++)
    {
        cout << a[i];
    }
    cout << endl;
    a.sort();
    for (int i = 0; i < 4; i++)
        cout << a[i];

    return 0;
}