132. Kth Smallest Element in a BST

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

Output: 1

Explanation: The inorder traversal is [1, 2, 3, 4]. The 1st smallest element is 1.

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

Output: 3

Explanation: The inorder traversal is [1, 2, 3, 4, 5, 6]. The 3rd smallest element is 3.

Time Complexity

O(n)

Space Complexity

O(h)