JavaAlgorithms/DataStructures/Stacks/StackOfLinkedList.java

package DataStructures.Stacks;

import java.util.NoSuchElementException;

/** @author Varun Upadhyay (https://github.com/varunu28) */

// An implementation of a Stack using a Linked List

class StackOfLinkedList {

  public static void main(String[] args) {

    LinkedListStack stack = new LinkedListStack();
    stack.push(1);
    stack.push(2);
    stack.push(3);
    stack.push(4);
    stack.push(5);

    System.out.println(stack);

    System.out.println("Size of stack currently is: " + stack.getSize());

    assert stack.pop() == 5;
    assert stack.pop() == 4;

    System.out.println("Top element of stack currently is: " + stack.peek());
  }
}

// A node class

class Node {
  public int data;
  public Node next;

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

/**
 * A class which implements a stack using a linked list
 *
 * <p>Contains all the stack methods : push, pop, printStack, isEmpty
 */
class LinkedListStack {

  /** Top of stack */
  Node head;

  /** Size of stack */
  private int size;

  /** Init properties */
  public LinkedListStack() {
    head = null;
    size = 0;
  }

  /**
   * Add element at top
   *
   * @param x to be added
   * @return <tt>true</tt> if add successfully
   */
  public boolean push(int x) {
    Node newNode = new Node(x);
    newNode.next = head;
    head = newNode;
    size++;
    return true;
  }

  /**
   * Pop element at top of stack
   *
   * @return element at top of stack
   * @throws NoSuchElementException if stack is empty
   */
  public int pop() {
    if (size == 0) {
      throw new NoSuchElementException("Empty stack. Nothing to pop");
    }
    Node destroy = head;
    head = head.next;
    int retValue = destroy.data;
    destroy = null; // clear to let GC do it's work
    size--;
    return retValue;
  }

  /**
   * Peek element at top of stack
   *
   * @return element at top of stack
   * @throws NoSuchElementException if stack is empty
   */
  public int peek() {
    if (size == 0) {
      throw new NoSuchElementException("Empty stack. Nothing to pop");
    }
    return head.data;
  }

  @Override
  public String toString() {
    Node cur = head;
    StringBuilder builder = new StringBuilder();
    while (cur != null) {
      builder.append(cur.data).append("->");
      cur = cur.next;
    }
    return builder.replace(builder.length() - 2, builder.length(), "").toString();
  }

  /**
   * Check if stack is empty
   *
   * @return <tt>true</tt> if stack is empty, otherwise <tt>false</tt>
   */
  public boolean isEmpty() {
    return size == 0;
  }

  /**
   * Return size of stack
   *
   * @return size of stack
   */
  public int getSize() {
    return size;
  }
}