// Stack.java// Implements a standard stack.// Note that you must check on your own if the stack is empty before trying to// pop an element.public class Stack{   public Node top, bottom;   public int numOfNodes;   public Stack()   {      top = null;      bottom = null;      numOfNodes = 0;   }   public boolean isEmpty()   {      return (top == null);   }   public void push(String s)   {      Node p = new Node(s);      p.next = top;      if (top != null)        top.prev = p;      top = p;      if (numOfNodes == 0)        bottom = p;      numOfNodes += 1;   }   public String pop()   {      Node p = top;      String s = p.data;      top = top.next;      if (top != null)        top.prev = null;      else        bottom = null;      p = null;      numOfNodes -= 1;      return s;   }   public String peek()   {      return top.data;   }   public void show()   {      Node p = top;      while (p != null)      {         System.out.println(p.data);         p = p.next;      }      System.out.println("");   }   public void erase()   {      top = null;      numOfNodes = 0;   }}