Kth Smallest Node in BST

def kthSmallest(root: TreeNode, k: int) -> int:
    cur = root
    stack = []
    counter = 1
    while cur or len(stack):
        while cur:
            stack.append(cur)
            cur = cur.left
        cur = stack.pop()
        if counter == k:
            return cur.val
        counter += 1
        cur = cur.right

    return None