Coursera Algorithms week4 基础标签表 练习测验：Inorder traversal with constant extra space

题目原文：

Design an algorithm to perform an inorder traversal of a binary search tree using only a constant amount of extra space.

public void traverse(BST<Key,Value> bst) {
    traverse(bst.root.left, bst.root);
}

private void traverse(Node current, Node parent) {
    while (current != null) {
        if (parent != null) {
            parent.left = current.right;
            current.right = parent;
        }
        if (current.left != null) {
            parent = current;
            current = current.left;
        } else {
            System.out.println(current.key);
            current = current.right;
            parent = null;
        }
    }
}