import java.util.Scanner;

interface Stack {
   public Object pop();
   public void push(Object x);
   public boolean empty();
   public int size();
}

class ArrayStack implements Stack {
   private Object data[];
   private int top;

   public ArrayStack() {
           data = new Object[1000];
           top = -1;
    }
    
    public int size() {
        return top + 1;
    }
    
    public boolean empty() {
        return top == -1;
    }
    
    public void push(Object x) {
        if (size() == 1000) 
           throw new RuntimeException("Stack Full");
        data[++top] = x;
    }
    
    public Object pop() {
        if (empty()) throw new RuntimeException("Stack Empty");
        return data[top--]; 
    }                      

// methods for testing purposes
   public String toString() {
      String ans = "Array Stack:  ";
      for (int i = top; i>= 0; i--)
         ans += ( data[i] + " -> ");
      return ans;
   }
   public static void main(String args[]) {
      Stack s = new ArrayStack();
      testStack(s);
   }
   public static void testStack(Stack s) {
      boolean done = false;
      while (!done) {
         try {
           Scanner sc = new Scanner(System.in);
           System.out.print( s + "  ");
           System.out.println("\ncmds are + - Q: >>");
           String cmd = sc.next();
           String entry = null;
           char command = cmd.charAt(0);
           if (command == '+') entry = sc.next();
           switch (cmd.charAt(0)) {
              case 'Q': done = true; break; 
              case '+': s.push(entry); break;
              case '-': s.pop(); break;
           }
        } catch (Exception e) {
           System.out.println("Error " + e.toString());
        }
      }   
   }
}

class ListAdapterStack implements Stack {
   private LinkedList l;
   public ListAdapterStack() {
      l = new LinkedList();
   }
   public int size() {
      return l.size();
   }
   public boolean empty() {
      return l.isEmpty();
   }
   public void push(Object d) {
      l.addHead(d);
   }
   public Object pop() {
      return l.removeHead();
   }
   public String toString() {
      return l.toString();
   }
   public static void main(String args[]) {
      Stack s = new ListAdapterStack();
      ArrayStack.testStack(s);
   }
}

class LinkedStack implements Stack
{
   private Node top;
   private int size;
   public LinkedStack() { 
      top = null; 
      size = 0; 
   }
   public int size() { 
      return size; 
   }
   public boolean empty() { 
      return size == 0; 
   }
   public Object pop() {
       if (empty()) throw new RuntimeException();
       Object answer = top.getData();
       top = top.getNext();
       size--;
       return answer;
   }
   public void push(Object x) {
       Node newtop = new Node(x, top);
       top = newtop;
       size++;
   }
   public String toString() {
      String ans = "Linked Stack:  ";
      Node n = top;
      while (n != null) {
         ans += ( n.getData() + " -> ");
         n = n.getNext();
      }
      return ans;
   }
   public static void main(String args[]) {
      Stack s = new LinkedStack();
      ArrayStack.testStack(s);
   }
}

class Node {
   private Object data;
   private Node next;
   public Node(Object x, Node n) { data = x; next = n; }
   public Object getData() { return data; }
   public Node getNext() { return next; }
   public void setData(Object x) { data = x; }
   public void setNext(Node n) { next = n; }
}