content: /*************************************************************************
   *  Compilation:  javac Stack.java
   *  Execution:    java Stack < input.txt
   *
   *  A generic stack, implemented using a linked list. Each stack
   *  element is of type Item.
   *  
   *  % more tobe.txt 
   *  to be or not to - be - - that - - - is
   *
   *  % java Stack < tobe.txt
   *  to be not that or be (2 left on stack)
   *
   *************************************************************************/
  
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  
  
  /**
   *  The <tt>Stack</tt> class represents a last-in-first-out (LIFO) stack of generic items.
   *  It supports the usual <em>push</em> and <em>pop</em> operations, along with methods
   *  for peeking at the top item, testing if the stack is empty, and iterating through
   *  the items in LIFO order.
   *  <p>
   *  All stack operations except iteration are constant time.
   *  <p>
   *  For additional documentation, see <a href="/algs4/13stacks">Section 1.3</a> of
   *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
   */
  public class Stack<Item> implements Iterable<Item> {
      private int N;          // size of the stack
      private Node first;     // top of stack
  
      // helper linked list class
      private class Node {
          private Item item;
          private Node next;
      }
  
     /**
       * Create an empty stack.
       */
      public Stack() {
          first = null;
          N = 0;
      }
  
     /**
       * Is the stack empty?
       */
      public boolean isEmpty() {
          return first == null;
      }
  
     /**
       * Return the number of items in the stack.
       */
      public int size() {
          return N;
      }
  
     /**
       * Add the item to the stack.
       */
      public void push(Item item) {
          Node oldfirst = first;
          first = new Node();
          first.item = item;
          first.next = oldfirst;
          N++;
      }
  
     /**
       * Delete and return the item most recently added to the stack.
       * Throw an exception if no such item exists because the stack is empty.
       */
      public Item pop() {
          if (isEmpty()) throw new RuntimeException("Stack underflow");
          Item item = first.item;        // save item to return
          first = first.next;            // delete first node
          N--;
          return item;                   // return the saved item
      }
  
  
     /**
       * Return the item most recently added to the stack.
       * Throw an exception if no such item exists because the stack is empty.
       */
      public Item peek() {
          if (isEmpty()) throw new RuntimeException("Stack underflow");
          return first.item;
      }
  
     /**
       * Return string representation.
       */
      public String toString() {
          StringBuilder s = new StringBuilder();
          for (Item item : this)
              s.append(item + " ");
          return s.toString();
      }
         
  
     /**
       * Return an iterator to the stack that iterates through the items in LIFO order.
       */
      public Iterator<Item> iterator()  { return new ListIterator();  }
  
      // an iterator, doesn't implement remove() since it's optional
      private class ListIterator implements Iterator<Item> {
          private Node current = first;
          public boolean hasNext()  { return current != null;                     }
          public void remove()      { throw new UnsupportedOperationException();  }
  
          public Item next() {
              if (!hasNext()) throw new NoSuchElementException();
              Item item = current.item;
              current = current.next; 
              return item;
          }
      }
  
  
     /**
       * A test client.
       */
      public static void main(String[] args) {
          Stack<String> s = new Stack<String>();
          while (!StdIn.isEmpty()) {
              String item = StdIn.readString();
              if (!item.equals("-")) s.push(item);
              else if (!s.isEmpty()) StdOut.print(s.pop() + " ");
          }
          StdOut.println("(" + s.size() + " left on stack)");
      }
  }
  
  