btinorderiterator.java

关于迭代器、构造器

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

/**
 * An iterator for traversing binary trees constructed from
 * BinaryTrees.  The iterator performs minimal work before
 * traversal.  Every node is considered after every left descendant,
 * but before any right descendant.  AbstractIterator finishes when
 * all descendants of the start node have been considered.
 * <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#inorderIterator() inorderIterator()};
 *      while (ti.{@link #hasNext() hasNext()})
 *      {
 *          System.out.println(ti.{@link #next() next()});
 *      }
 *      ti.{@link #reset() reset()};
 *      while (ti.{@link #hasNext() hasNext()})
 *      { .... }
 * </pre>
 *
 * @version $Id: BTInorderIterator.java,v 4.0 2000/12/27 20:57:33 bailey Exp bailey $
 * @author, 2001 duane a. bailey
 */
class BTInorderIterator extends AbstractIterator
{
    /**
     * The root of the subtree being traversed.
     */
    protected BinaryTree root; // root of subtree to be traversed
    /** 
     * Stack of nodes that maintain the state of the iterator.
     */
    protected Stack todo; // stack of unvisited ancestors of current

    /**
     * Construct a new inorder iterator of a tree.
     *
     * @post Constructs an iterator to traverse inorder
     * 
     * @param root The root of the subtree to be traversed.
     */
    public BTInorderIterator(BinaryTree root)
    {
	todo = new StackList();
	this.root = root;
	reset();
    }	

    /**
     * Reset the iterator to its initial state.
     *
     * @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 != BinaryTree.EMPTY) {
	    todo.push(current);
	    current = current.left();
	}
    }

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

    /**
     * Return the node currently being considered.
     *
     * @pre hasNext()
     * @post Returns reference to current value
     * 
     * @return The node currently under consideration.
     */
    public Object get()
    {	
	return ((BinaryTree)todo.getFirst()).value();
    }

    /**
     * Return current node, and increment iterator.
     *
     * @pre hasNext()
     * @post Returns current value, increments iterator
     * @return The value of the current node, before iterator iterated.
     */
    public Object next()
    {
	BinaryTree old = (BinaryTree)todo.pop();
	Object result = old.value();
	// we know this node has no unconsidered left children;
	// if this node has a right child, 
	//   we push the right child and its leftmost descendants:
	// else 
        //   top element of stack is next node to be visited
	if (!old.right().isEmpty()) {
	    BinaryTree current = old.right();
	    do {
		todo.push(current);
		current = current.left();
	    } while (!current.isEmpty());
	}
	return result;
    }
}