Given the root node of a valid BST and a number k, return the kth smallest number in this BST (1-indexed).
Input
bst: a binary tree representing the existing BST.
k: an integer.
Output
The kth smallest number in bst.
Examples
Example 1
Input
bst = <See explanation>
k = 4
Output: 6
Explanation
Constraints
1 <= k <= n <= 10^5, where n is the size of bst.