Building my Own Stack

• Explained

import static java.lang.System.exit;

// Driver code
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
		System.out.printf("\nTop element is %d\n",
						obj.peek());

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

		// print Stack elements
		obj.display();

		// print Top element of Stack
		System.out.printf("\nTop element is %d\n",
						obj.peek());
	}
}

// Create Stack Using Linked list
class StackUsingLinkedlist {

	// A linked list node
	private class Node {

		int data; // integer data
		Node link; // reference variable Node type
	}
	// create global top reference variable global
	Node top;
	// Constructor
	StackUsingLinkedlist() { this.top = null; }

	// Utility function to add an element x in the stack
	public void push(int x) // insert at the beginning
	{
		// 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) {
			System.out.print("\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 boolean 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 {
			System.out.println("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) {
			System.out.print("\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) {
			System.out.printf("\nStack Underflow");
			exit(1);
		}
		else {
			Node temp = top;
			while (temp != null) {

				// print node data
				System.out.print(temp.data);

				// assign temp link to temp
				temp = temp.link;
				if(temp != null)
					System.out.print(" -> ");
			}
		}
	}
}

GFG.main(null);

44 -> 33 -> 22 -> 11
Top element is 44
22 -> 11
Top element is 22

Explained

• defines a class StackUsingLinkedlist that has methods to perform basic stack operations like push, pop, peek, and display
• The push method inserts a new element into the stack by creating a new node with the specified data, and making it the top of the stack by updating the top reference variable.
• The pop method removes the top element from the stack by updating the top reference variable to point to the next node in the linked list.
• The peek method returns the value of the top element in the stack without removing it.
• The display method prints the elements of the stack starting from the top element.
• The main method creates an object of the StackUsingLinkedlist class, inserts some elements into the stack, displays the elements, performs some stack operations like pop and peek, and displays the stack again to show the changes.
• The main function creates an object of the StackUsingLinkedList class, inserts four elements into the stack using push, displays all the elements in the stack using display, prints the top element using peek, and removes two elements from the top of the stack using pop. Finally, it displays the remaining elements in the stack using display and prints the top element using peek