Stack_with_operations_on_middle_element.cpp

#include <bits/stdc++.h>
using namespace std;

class Node
{
public:
    int data;
    Node *prev, *next;
    Node()
    {
        prev = next = NULL;
    }
};

class Stack : public Node
{
private:
    Node *head;
    Node *middle;
    int counter;

public:
    Stack()
    {
        head = middle = NULL;
        counter = 0;
    }

    bool isEmpty()
    {
        if (head == NULL)
            return true;
        return false;
    }

    void push(int data)
    {
        counter++;
        Node *node = new Node();
        node->data = data;
        cout << data << " is pushed in." << endl;
        node->prev = NULL;
        node->next = head;

        if (head != NULL)
            head->prev = node;

        head = node;

        if (counter == 1)
            middle = node;
        else
        {
            if (!(counter & 1))
                middle = middle->prev;
        }
    }

    void pop()
    {
        if (counter == 0 || isEmpty())
            cout << "\nStack is empty." << endl;

        Node *temp = head;
        head = head->next;

        if (head != NULL)
            head->prev = NULL;

        counter--;

        // if counter is odd.
        if (counter & 1)
            middle = middle->next;

        cout << temp->data << " is popped out." << endl;

        free(temp);
    }

    void middleElement()
    {
        if (counter == 0)
            cout << "\nStack is empty." << endl;

        cout << "\nMiddle element is : " << middle->data << endl;
    }

    void display()
    {
        Node *temp;
        while (temp->next != NULL)
        {
            cout << temp->data;
            temp = temp->next;
        }
        cout << endl;
    }
};


int main()
{
    Stack s;
    s.push(11);
    s.push(22);
    s.push(33);
    s.push(44);
    s.push(55);
    s.push(66);
    s.push(77);

    s.middleElement();

    s.pop();
    s.pop();
    s.pop();

    s.middleElement();

    return 0;
}