173. 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.

173：next都O（1）貌似实现不了吧

# Definition for a  binary tree node
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class BSTIterator(object):
    def __init__(self, root):
        """
        :type root: TreeNode
        """
        self.st = []
        while root:
            self.st.append(root)
            root = root.left

    def hasNext(self):
        """
        :rtype: bool
        """
        return self.st

    def next(self):
        """
        :rtype: int
        """
        res = self.st.pop()
        root = res.right
        while root:
            self.st.append(root)
            root = root.left
        return res.val

# Your BSTIterator will be called like this:
# i, v = BSTIterator(root), []
# while i.hasNext(): v.append(i.next())