closest-binary-search-tree-value-ii

Given the root of a binary search tree, a target value, and an integer k, return the k values in the BST that are closest to the target. You may return the answer in any order.

You are guaranteed to have only one unique set of k values in the BST that are closest to the target.

Input: root = [4,2,5,1,3], target = 3.714286, k = 2
Output: [4,3]


Solution:
time: O(N log k) 
space O(K+h)

Instantiate the heap with "less close element first" strategy so that the heap contains the elements that are closest to the target.

Use inorder traversal to traverse the tree following the direction Left -> Node -> Right.

Push all elements into heap during the traversal, keeping the heap size less than or equal to k.
As a result, the heap contains k elements that are closest to target. Convert it into a list and return.