Binary Search Tree Iterator

Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.

Calling next() will return the next smallest number in the BST.

Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

问题描述：BST的Iterator,next()从小到大依次给出，next(),hasNext() O(1)时间复杂度 ,O(h)内存高度；

1:初始化时一次性中序遍历到arraylist中（体现不出O(h)要求）：

2：初始化把树的最左排的节点依次压入stack中，hasNext() stack.isEmpty(),当调用next()时，在把目前节点的右子树的左排节点依次压入