new_contents: """Leetcode 173. Binary Search Tree Iterator
  Medium
  
  URL: https://leetcode.com/problems/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.
  
  Example:
      7
     / \
    3  15
      /  \
     9   20  
  BSTIterator iterator = new BSTIterator(root);
  iterator.next();    // return 3
  iterator.next();    // return 7
  iterator.hasNext(); // return true
  iterator.next();    // return 9
  iterator.hasNext(); // return true
  iterator.next();    // return 15
  iterator.hasNext(); // return true
  iterator.next();    // return 20
  iterator.hasNext(); // return false
  
  Note:
  - next() and hasNext() should run in average O(1) time and uses O(h) memory, 
    where h is the height of the tree.
  - You may assume that next() call will always be valid, that is, there will be
    at least a next smallest number in the BST when next() is called.
  """
  
  # Definition for a binary tree node.
  class TreeNode(object):
      def __init__(self, val):
          self.val = val
          self.left = None
          self.right = None
  
  class BSTIterator(object):
  
      def __init__(self, root):
          """
          :type root: TreeNode
          """
          pass
  
      def next(self):
          """
          @return the next smallest number
          :rtype: int
          """
          pass
  
      def hasNext(self):
          """
          @return whether we have a next smallest number
          :rtype: bool
          """
          pass
  
  
  def main():
      pass
  
  
  if __name__ == '__main__':
      main()
  
subject: Add lc0173_binary_search_tree_iterator.py