25.10 (Preorder iterator) Add the following method in the BST class that returns an iterator for
traversing the elements in a BST in preorder. /** Returns an iterator for traversing the elements
in preorder */ java.util.IteratorpreorderIterator()
Solution
class BTPreorderIterator extends AbstractIterator { /** * The root of the subtree to be considered
by traversal. */ protected BinaryTree root; // root of tree to be traversed /** * The stack that
maintains the state of the iterator. */ protected Stack > todo; // stack of unvisited nodes whose /**
* Constructs a pre-order traversal of subtree rooted at root. * * @post constructs an iterator to
traverse in preorder * * @param root Root of subtree to be traversed. */ public
BTPreorderIterator(BinaryTree root) { todo = new StackList >(); this.root = root; reset(); } /** *
Resets 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 root if (root != null) todo.push(root);
} /** * Returns true if some nodes of subtree have yet to be considered. * * @post returns true iff
iterator is not finished * * @return True iff more nodes to be considered in traversal. */ public
boolean hasNext() { return !todo.isEmpty(); } /** * Returns the value currently being referenced
by iterator. * * @pre hasNext() * @post returns reference to current value * * @return The
current value. */ public E get() { return todo.get().value(); } /** * Returns the current value and
increments the iterator. * Iterator is then incremented. * * @pre hasNext(); * @post returns
current value, increments iterator * * @return The value currently being considered. */ public E
next() { BinaryTree old = todo.pop(); E result = old.value(); if (!old.right().isEmpty())
todo.push(old.right()); if (!old.left().isEmpty()) todo.push(old.left()); return result; } }