btpostorderiterator.java

关于迭代器、构造器

// Post-order iterator for binary trees.
// (c) 1998, 2001 duane a. bailey
package structure;

/**
 * This class implements a post-order traversal of a binary tree.
 * This iterator considers every node after its non-trivial descendants.
 * <P>
 * Example usage:
 * <P>
 * <pre>
 *      {@link structure.BinaryTree BinaryTree} t = new {@link structure.BinaryTree#BinaryTree() BinaryTree()};
 *      // ...tree is grown
 *      {@link java.util.Iterator Iterator} ti = t.{@link structure.BinaryTree#postorderIterator() postorderIterator()};
 *      while (ti.{@link #hasNext() hasNext()})
 *      {
 *          System.out.println(ti.{@link #next() next()});
 *      }
 *      ti.{@link #reset() reset()};
 *      while (ti.{@link #hasNext() hasNext()})
 *      { .... }
 * </pre>
 *
 * @see BinaryTree#PostorderElements
 * @version $Id: BTPostorderIterator.java,v 4.0 2000/12/27 20:57:33 bailey Exp bailey $
 * @author, 2001 duane a. bailey
 */
class BTPostorderIterator extends AbstractIterator
{
    /**
     * The root of the tree currently being traversed.
     */
    protected BinaryTree root; // root of traversed subtree
    /**
     * The stack the maintains the state of the iterator.
     * Elements of the stack are nodes whose descendants are still being
     * considered.
     */
    protected Stack todo;  // stack of nodes whose descendants
                           // are currently being visited

    /**
     * Construct an iterator to traverse subtree rooted at root
     * in post-order.
     *
     * @post constructs an iterator to traverse in postorder
     * 
     * @param root The root of the subtree to be traversed.
     */
    public BTPostorderIterator(BinaryTree root)
    {
	todo = new StackList();
	this.root = root;
	reset();
    }	

    /**
     * Reset the iterator to the first node of the traversal.
     *
     * @post Resets the iterator to retraverse
     */
    public void reset()
    {
	todo.clear();
	// stack is empty; push on nodes from root to
	// leftmost descendant
	BinaryTree current = root;
	while (!current.isEmpty()) {
	    todo.push(current);
	    if (!current.left().isEmpty())
		current = current.left();
	    else
		current = current.right();
	}
    }

    /**
     * Return true iff more nodes are to be considered in traversal.
     *
     * @post Returns true iff iterator is not finished
     * 
     * @return True iff more nodes are to be considered in traversal.
     */
    public boolean hasNext()
    {
	return !todo.isEmpty();
    }

    /**
     * Return the value of the current node.
     *
     * @pre hasNext()
     * @post returns reference to current value
     * 
     * @return The value referenced by current node.
     */
    public Object get()
    {	
	return ((BinaryTree)todo.getFirst()).value();
    }

    /**
     * Return current value and increment iterator.
     *
     * @pre hasNext();
     * @post returns current value, increments iterator
     * 
     * @return The value currently considered by iterator, increment.
     */
    public Object next()
    {
	BinaryTree current = (BinaryTree)todo.pop();
	Object result = current.value();
	if (!todo.isEmpty())
	{
	    BinaryTree parent = (BinaryTree)todo.getFirst();
	    if (current == parent.left()) {
		current = parent.right();
		while (!current.isEmpty())
		{
		    todo.push(current);
		    if (!current.left().isEmpty())
			 current = current.left();
		    else current = current.right();
		}
	    }
	}
	return result;
    }
}