Kth Smallest Element in a BST

Given the root of a binary search tree, and an integer k, return the kth smallest value (1-indexed) in the tree. A binary search tree satisfies the following constraints:

• The left subtree of every node contains only nodes with keys less than the node's key.
• The right subtree of every node contains only nodes with keys greater than the node's key.
• Both the left and right subtrees are also binary search trees.

Interview note: A common follow-up asks for a solution with O(1) extra space.