import java.util.Iterator;
import java.util.NoSuchElementException;
import structure5.*;

/**
 * Implements an in-order traversal of a BinarySearchTree.
 * Implementing this iterator is much easier if you use the
 * BST in Bailey, Ch. 13, which keeps track of parents.
 * Since we don't have parents available, we instead use
 * a Stack.  Alternatively, we could have written this
 * recursively using a helper method.
 */
public class BinarySearchTreeIterator<T extends Comparable<T>> implements Iterator<T> {
    BinarySearchTree<T> bst;
    BinarySearchTree<T> cur;
    Stack<BinarySearchTree<T>> parents;

    public BinarySearchTreeIterator(BinarySearchTree<T> bst) {
        this.bst = bst;
        this.cur = null;
        parents = new StackList<BinarySearchTree<T>>();

        // set cur to the leftmost node
        cur = bst;
        if (cur == null) return;
        while(cur.getLeft() != null) {
            parents.add(cur);
            cur = cur.getLeft();
        }
    }
    
    public boolean hasNext() {
        return cur != null;
    }

    public T next() {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }
    
        // invariant: cur is always the leftmost node
        BinarySearchTree<T> tmp = cur;
    
        // if there is a right side, go right, then
        // go as far left as possible
        if (cur.getRight() != null) {
            parents.add(cur);
            cur = cur.getRight();
            // get leftmost node
            while (cur.getLeft() != null) {
                parents.add(cur);
                cur = cur.getLeft();
            }
        // no right child; find the first parent from the left
        } else if (parents.isEmpty()) {
            cur = null;  // done
        } else {
            // keep popping until we come from the left
            BinarySearchTree<T> prev = cur;
            while(true) {
                cur = parents.pop();
                if (cur.getLeft() == prev) {
                    break;
                }
                prev = cur;
            }
        }
        // return the current value
        return tmp.getValue();
    }

    public void remove() {
        // do nothing for now
    }
}