Kth Smallest Element in a BST

Problem Return the kth smallest value in a binary search tree.

Input / Output

• Input: the root of a BST and an integer k.
• Output: the kth smallest node value (1-indexed).

Constraints

• 1 <= k <= number of nodes; values are distinct.

Example

• BST {3,1,4,null,2}, k = 1 → 1; k = 3 → 3.