public interface Stack { // Stack class ADT
  // Reinitialize the stack.
  public void clear();

  // Push "it" onto the top of the stack
  public boolean push(Object it);

  // Remove and return the element at the top of the stack
  public Object pop();

  // Return a copy of the top element
  public Object topValue();

  // Return the number of elements in the stack
  public int length();
  
  // Return true if the stack is empty 
  public boolean isEmpty();
}

interface Stack { // Stack class ADT
  // Reinitialize the stack.
  void clear();

  // Push "it" onto the top of the stack
  boolean push(Object it);

  // Remove and return the element at the top of the stack
  Object pop();

  // Return a copy of the top element
  Object topValue();

  // Return the number of elements in the stack
  int length();
}

public interface Stack<E> { // Stack class ADT
  // Reinitialize the stack.
  public void clear();

  // Push "it" onto the top of the stack
  public boolean push(E it);

  // Remove and return the element at the top of the stack
  public E pop();

  // Return a copy of the top element
  public E topValue();

  // Return the number of elements in the stack
  public int length();
  
  // Tell if the stack is empty or not
  public boolean isEmpty();
}

class AStack implements Stack {
  private Object stackArray[];    // Array holding stack
  private static final int DEFAULT_SIZE = 10;
  private int maxSize;            // Maximum size of stack
  private int top;                // First free position at top

  // Constructors
  AStack(int size) {
    maxSize = size;
    top = 0;
    stackArray = new Object[size]; // Create stackArray
  }
  AStack() { this(DEFAULT_SIZE); }

class AStack implements Stack {
  private Object stackArray[];    // Array holding stack
  private static final int DEFAULT_SIZE = 10;
  private int maxSize;            // Maximum size of stack
  private int top;                // First free position at top

  // Constructors
  AStack(int size) {
    maxSize = size;
    top = 0;
    stackArray = new Object[size]; // Create stackArray
  }
  AStack() { this(DEFAULT_SIZE); }

class AStack<E> implements Stack<E> {
  private E stackArray[];         // Array holding stack
  private static final int DEFAULT_SIZE = 10;
  private int maxSize;            // Maximum size of stack
  private int top;                // First free position at top

  // Constructors
  @SuppressWarnings("unchecked") // Generic array allocation
  AStack(int size) {
    maxSize = size;
    top = 0;
    stackArray = (E[])new Object[size]; // Create stackArray
  }
  AStack() { this(DEFAULT_SIZE); }

// Linked stack implementation
class LStack implements Stack {
  private Link top;               // Pointer to first element
  private int size;               // Number of elements

  // Constructors
  LStack() { top = null; size = 0; }
  LStack(int size) { top = null; size = 0; }

// Linked stack implementation
class LStack implements Stack {
  private Link top;               // Pointer to first element
  private int size;               // Number of elements

  // Constructors
  LStack() { top = null; size = 0; }
  LStack(int size) { top = null; size = 0; }

// Linked stack implementation
class LStack<E> implements Stack<E> {
  private Link<E> top;            // Pointer to first element
  private int size;               // Number of elements

  // Constructors
  LStack() { top = null; size = 0; }
  LStack(int size) { top = null; size = 0; }