import java.util.NoSuchElementException;

public class ListStack<T> implements MyStack<T>
{
   /*******************************************************
     The private inner class "Node" 
    *******************************************************/
   // make Node class name of generic type
   private class Node<T>
   {
      // Node instance variable is generic
      private T item;
      private Node<T> next;

      // parameters in constructor generic
      public Node(T item, Node<T> next)
      {
         this.item = item;
         this.next = next;
      }

      public String toString(){ return "" + this.item; }
   } // End of private inner class "Node"

   private Node<T> first;

   // Constructs an empty list
   public ListStack()
   {
      first = null;
   }

   // Returns true if the listStack is empty
   public boolean isEmpty()
   {
      return first==null;
   }

   // Returns true if the listStack is full
   public boolean isFull()
   {
      return false;    // A linked list is never full...
   }

   // push(item) --> insert at the front of the list
   public void push(T item)
   {
      Node<T> newNode = new Node<T>(item, first);
      first = newNode;
   }

   // pop() --> remove the first item and return it
   public T pop()
   {
      if(isEmpty())
      {
         throw new NoSuchElementException();
      }

      T toReturn = first.item;
      first = first.next;
      return toReturn;
   }

   // peek()
   public T peek()
   {
      if(isEmpty())
      {
         throw new NoSuchElementException();
      }

      return first.item;
   }

   // Returns a string representation
   public String toString()
   {
      String output = "";

      if(first == null) 
         return "[NULL]";

      Node<T> tmp = first;
      while(tmp != null) 
      {
         output += tmp + " -> ";
         tmp = tmp.next;
      }
      output += "[NULL]";
      return output;
   }

} // End of class