implement an iterator over a binary search tree (bst). your iterator will be initialized with the root node of a bst.
calling <code>next()</code> will return the next smallest number in the bst.
note: <code>next()</code> and <code>hasnext()</code> should
run in average o(1) time and uses o(h) memory, where h is the height of the tree.
中序遍历
