/media/sda-magnetic/david/Dokumente-15/fernuni-hagen/cs-i-ii/old-cs-2-03/java-src/Stackprogram.java

class Node1 {
  Node1 next;
  Node1 prev;
  String data;

}


class Stack1 {
   Node1 head; 
   Node1 ptr;
   Node1 tmp;
   
   Stack1 () {
     head = null;
     ptr = head;
   }

   void push (String data) {
      if (head == null) {
        head = new Node1();
        head.next = null;
        head.prev = head;
        head.data = data;
        ptr = head;
      }
      else {
        tmp = new Node1();
        ptr.next = tmp;
        tmp.prev = ptr;
        tmp.next = null;
        tmp.data = data;
        ptr = ptr.next;
      }
   }
   
   String pop () {
      String data;
      
      try {
        data = ptr.data;
      }
      catch (java.lang.NullPointerException ex) {
         System.out.println ("+++ Empty Stack +++  Empty Stack +++ Empty Stack +++");
      }
      if (ptr != head) {
         data = ptr.data;
         ptr = ptr.prev;
         ptr.next = null;
         return data;      
      }
      else {
         data = ptr.data;
         ptr = null;
         head = null;
         return data;
      }
   }
   
   boolean isEmpty () {
     return (head == null);
   }
   
   Node1 peek() {
      return ptr;
   }
}


public class Stackprogram {

   public static void main(String [] args) {
     Stack1 stck = new Stack1();
     
     stck.push ("David");
     stck.push ("Vajda");
     stck.push ("Anton");
     stck.push ("Berta");
     stck.push ("Ceasar");
     stck.push ("Dora");
     
     System.out.println (stck.pop());
     System.out.println (stck.pop());
     System.out.println (stck.pop());
     System.out.println (stck.pop());
     
     stck.push ("Emil");
     stck.push ("Friedrich");
     stck.push ("Gustav");
     
     System.out.println (stck.pop());
     System.out.println (stck.pop());
     System.out.println (stck.pop());
     if (stck.isEmpty())
       System.out.println ("Stack is empty");
     else 
       System.out.println ("Stack is not empty");
     System.out.println (stck.pop());
     System.out.println (stck.pop());
     if (stck.isEmpty())
       System.out.println ("Stack is empty");
     else 
       System.out.println ("Stack is not empty");
     
     System.out.println (stck.pop());
     System.out.println (stck.pop());
   
   
   }
}