173. Binary Search Tree Inorder Iterator

Design a class InorderBSTIterator that iterates over the nodes of a binary search tree (BST) in in-order traversal. The iterator must support the following operations: (1) InorderBSTIterator(TreeNode root): initialize the iterator with the root node of the BST. (2) has_next(): return True if there is a next node in the in-order traversal, otherwise False. (3) get_next(): return the next node value in the in-order traversal. The first call to get_next() returns the smallest value in the BST. You may assume get_next() is only called when has_next() is True. The TreeNode class is defined as: class TreeNode: def __init__(self, val=0, left=None, right=None): self.val = val self.left = left self.right = right