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Kth Smallest Element in a BST

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LeetCode 230. Kth Smallest Element in a BST

Given the root of a binary search tree and an integer k, return the k^(th) smallest value (1-indexed) of all the values of the nodes in the tree.

Examples

Example 1

Input:  root = [3,1,4,null,2], k = 1

3 / \ 1 4 \ 2Output: 1

Example 2

Input:  root = [5,3,6,2,4,null,null,1], k = 3

5 / \ 3 6 / \ 2 4 / 1Output: 3

Constraints

  • The number of nodes in the tree is n
  • 1 <= k <= n <= 10⁴
  • 0 <= Node.val <= 10⁴

How the tests work: trees are written as level-order arrays — [1,null,2] is a root 1 with a right child 2. Your function receives real TreeNode objects.

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List of test cases (8)
Case 1
root = [3,1,4,null,2]
k = 1
Case 2
root = [5,3,6,2,4,null,null,1]
k = 3
Case 3
root = [1]
k = 1
Case 4
root = [3,1,4,null,2]
k = 2
Case 5
root = [5,3,6,2,4,null,null,1]
k = 5
Case 6
root = [5,3,6,2,4,null,null,1]
k = 1
Case 7
root = [10,5,15,3,7,12,20]
k = 4
Case 8
root = [4,2,6,1,3,5,7]
k = 7