Given the `root` of a binary search tree, and an integer `k`, return the `kth` smallest value (1-indexed) of all the values of the nodes in the tree.

Example 1:

    Input: root = [3,1,4,null,2], k = 1
    Output: 1

Example 2:

    Input: root = [5,3,6,2,4,null,null,1], k = 3
    Output: 3

Constraints:

- The number of nodes in the tree is `n`.
- `1 <= k <= n <= 104`
- `0 <= Node.val <= 104`

Follow up: If the BST is modified often (i.e., we can do insert and delete operations) and you need to find the kth smallest frequently, how would you optimize?