272. Closest Binary Search Tree Value II

Given a non-empty binary search tree and a target value, find k values in the BST that are closest to the target.

Note:

• Given target value is a floating point.
• You may assume k is always valid, that is: k ≤ total nodes.
• You are guaranteed to have only one unique set of k values in the BST that are closest to the target.

Example:

Input:

 root = [4,2,5,1,3], target = 3.714286, and 
k

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

 [4,3]

Follow up:
Assume that the BST is balanced, could you solve it in less than O ( n ) runtime (where n = total nodes)