content: /*************************************************************************
   *  Compilation:  javac Bag.java
   *  Execution:    java Bag < input.txt
   *
   *  A generic bag or multiset, implemented using a linked list.
   *
   *************************************************************************/
  
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  
  /**
   *  The <tt>Bag</tt> class represents a bag (or multiset) of 
   *  generic items. It supports insertion and iterating over the 
   *  items in arbitrary order.
   *  <p>
   *  The <em>add</em>, <em>isEmpty</em>, and <em>size</em>  operation 
   *  take constant time. Iteration takes time proportional to the number of items.
   *  <p>
   *  For additional documentation, see <a href="http://algs4.cs.princeton.edu/13stacks">Section 1.3</a> of
   *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
   */
  public class Bag<Item> implements Iterable<Item> {
      private int N;         // number of elements in bag
      private Node first;    // beginning of bag
  
      // helper linked list class
      private class Node {
          private Item item;
          private Node next;
      }
  
     /**
       * Create an empty stack.
       */
      public Bag() {
          first = null;
          N = 0;
      }
  
     /**
       * Is the BAG empty?
       */
      public boolean isEmpty() {
          return first == null;
      }
  
     /**
       * Return the number of items in the bag.
       */
      public int size() {
          return N;
      }
  
     /**
       * Add the item to the bag.
       */
      public void add(Item item) {
          Node oldfirst = first;
          first = new Node();
          first.item = item;
          first.next = oldfirst;
          N++;
      }
  
  
     /**
       * Return an iterator that iterates over the items in the bag.
       */
      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;
          }
      }
  
  }
  