```
# 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.stack = []
node = root
while node:
self.stack.append(node)
node = node.left
def hasNext(self):
"""
:rtype: bool
"""
return self.stack
def next(self):
"""
:rtype: int
"""
node = self.stack.pop()
res = node.val
node = node.right
while node:
self.stack.append(node)
node = node.left
return res
# Your BSTIterator will be called like this:
# i, v = BSTIterator(root), []
# while i.hasNext(): v.append(i.next())
```