root of a binary search tree, and an integer
k, return the
kth (1-indexed) smallest element in the tree.
Input: root = [3,1,4,null,2], k = 1 Output: 1
Input: root = [5,3,6,2,4,null,null,1], k = 3 Output: 3
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?