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) of all the values of the nodes in the tree.

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Example 1

  3
 / \
1   4
 \
  2

Input: root = [3, 1, 4, null, 2], k = 1
Output: 1
Explanation: The in-order traversal is [1, 2, 3, 4]. The 1st smallest element is 1.

Example 2

      5
     / \
    3   6
   / \
  2   4
 /
1

Input: root = [5, 3, 6, 2, 4, null, null, 1], k = 3
Output: 3
Explanation: The in-order traversal is [1, 2, 3, 4, 5, 6]. The 3rd smallest element is 3.

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Constraints

• The number of nodes in the tree is n.
• 1 <= k <= n <= 10^4
• 0 <= Node.val <= 10^4