Validate BST

Question (LC.230)

Given the root of a BST, find the kth smallest element.

Example

I: tree = [3,1,4,null,2], k = 1

   3
  / \
 1   4
  \
   2

O: 1

I: tree = [5,3,6,2,4,null,null,1], k = 3
       5
      / \
     3   6
    / \
   2   4
  /
 1

O: 3