Binary Search Tree Iterator

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

class BSTIterator(object):

    def __init__(self, root):
        """
        :type root: Optional[TreeNode]
        """
        self.stack = []
        self._leftmost_inorder(root)

    def _leftmost_inorder(self, root):
        """
        Helper function to traverse the leftmost path of the tree.
        """
        while root:
            self.stack.append(root)
            root = root.left

    def next(self):
        """
        :rtype: int
        """
        # Pop the top element from the stack (smallest available value)
        topmost_node = self.stack.pop()
        # If the node has a right child, traverse its leftmost subtree
        if topmost_node.right:
            self._leftmost_inorder(topmost_node.right)
        return topmost_node.val

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


# Your BSTIterator object will be instantiated and called as such:
# obj = BSTIterator(root)
# param_1 = obj.next()
# param_2 = obj.hasNext()