Representation of a Stack as a Linked List in Java

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import java.util.Scanner;

public class StackUsingLinkedList {

    // Node class for the linked list
    private static class Node {
        int data;
        Node next;

        Node(int data) {
            this.data = data;
            this.next = null;
        }
    }

    private Node top;     // Top of stack
    private int size;     // Optional: track number of elements

    public StackUsingLinkedList() {
        this.top = null;
        this.size = 0;
    }

    // Push operation: O(1)
    public void push(int value) {
        Node newNode = new Node(value);
        newNode.next = top;
        top = newNode;
        size++;
    }

    // Pop operation: O(1)
    public int pop() {
        if (isEmpty()) {
            System.out.println("Stack Underflow: Cannot pop from an empty stack.");
            return Integer.MIN_VALUE;
        }
        int poppedValue = top.data;
        top = top.next;
        size--;
        return poppedValue;
    }

    // Peek operation: O(1)
    public int peek() {
        if (isEmpty()) {
            System.out.println("Stack is empty: Nothing to peek.");
            return Integer.MIN_VALUE;
        }
        return top.data;
    }

    // isEmpty operation: O(1)
    public boolean isEmpty() {
        return top == null;
    }

    // size operation: O(1)
    public int size() {
        return size;
    }

    // Display stack elements from top to bottom: O(n)
    public void display() {
        if (isEmpty()) {
            System.out.println("Stack is empty.");
            return;
        }

        System.out.print("Stack (top -> bottom): ");
        Node current = top;
        while (current != null) {
            System.out.print(current.data);
            if (current.next != null) System.out.print(" -> ");
            current = current.next;
        }
        System.out.println();
    }

    // Demo main
    public static void main(String[] args) {
        StackUsingLinkedList stack = new StackUsingLinkedList();

        // Example: fixed set of operations (simple beginner demo)
        stack.push(10);
        stack.push(20);
        stack.push(30);

        stack.display();

        System.out.println("Peek: " + stack.peek());
        System.out.println("Pop: " + stack.pop());
        System.out.println("Pop: " + stack.pop());

        stack.display();
        System.out.println("Current size: " + stack.size());

        // Optional: show pop on empty behavior quickly
        System.out.println("Pop: " + stack.pop());
        System.out.println("Pop: " + stack.pop()); // underflow example
    }
}