# Leetcode: Binary Search Tree Iterator

Binary Search Tree Iterator

Similar Problems:

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.

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

```## Blog link: https://code.dennyzhang.com/binary-search-tree-iterator
## Basic Ideas: In-order traversal. Use a stack
##      Store directed left children from root.
##      When next() be called, pop one element and process its right child as new root.
## Complexity:
##
# 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 = []
p = root
while p:
self.stack.append(p)
p = p.left

def hasNext(self):
"""
:rtype: bool
"""
return len(self.stack) > 0

def next(self):
"""
:rtype: int
"""
node = self.stack.pop()
p = node.right
while p:
self.stack.append(p)
p = p.left
return node.val

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

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