Kth Smallest in BST

1def kthSmallest(root: Optional[TreeNode], k: int) -> int:
2    stack = []
3    cur = root
4    while cur or stack:
5        while cur:
6            stack.append(cur)
7            cur = cur.left
8        cur = stack.pop()
9        k -= 1
10        if k == 0:
11            return cur.val
12        cur = cur.right
13    return -1