using System;

// Create Stack Using Linked list
public class StackUsingLinkedlist {

    // A linked list node
    private class Node {
        // integer data
        public int data;

        // reference variable Node type
        public Node link;
    }

    // create global top reference variable
    Node top;

    // Constructor
    public StackUsingLinkedlist() { this.top = null; }

    // Utility function to add
    // an element x in the stack
    // insert at the beginning
    public void push(int x)
    {
        // create new node temp and allocate memory
        Node temp = new Node();

        // check if stack (heap) is full.
        // Then inserting an element
        // would lead to stack overflow
        if (temp == null) {
            Console.Write("\nHeap Overflow");
            return;
        }

        // initialize data into temp data field
        temp.data = x;

        // put top reference into temp link
        temp.link = top;

        // update top reference
        top = temp;
    }

    // Utility function to check if
    // the stack is empty or not
    public bool isEmpty() { return top == null; }

    // Utility function to return
    // top element in a stack
    public int peek()
    {
        // check for empty stack
        if (!isEmpty()) {
            return top.data;
        }
        else {
            Console.WriteLine("Stack is empty");
            return -1;
        }
    }

    // Utility function to pop top element from the stack
    public void pop() // remove at the beginning
    {
        // check for stack underflow
        if (top == null) {
            Console.Write("\nStack Underflow");
            return;
        }

        // update the top pointer to
        // point to the next node
        top = (top).link;
    }

    public void display()
    {
        // check for stack underflow
        if (top == null) {
            Console.Write("\nStack Underflow");
            return;
        }
        else {
            Node temp = top;
            while (temp != null) {

                // print node data
                Console.Write(temp.data);

                // assign temp link to temp
                temp = temp.link;
                if(temp != null)
                    Console.Write(" -> ");
            }
        }
    }
}

// Driver code
public class GFG {
    public static void Main(String[] args)
    {
        // create Object of Implementing class
        StackUsingLinkedlist obj
            = new StackUsingLinkedlist();

        // insert Stack value
        obj.push(11);
        obj.push(22);
        obj.push(33);
        obj.push(44);

        // print Stack elements
        obj.display();

        // print Top element of Stack
        Console.Write("\nTop element is {0}\n", obj.peek());

        // Delete top element of Stack
        obj.pop();
        obj.pop();

        // print Stack elements
        obj.display();

        // print Top element of Stack
        Console.Write("\nTop element is {0}\n", obj.peek());
    }
}