270. Closest Binary Search Tree Value

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

Note:

• Given target value is a floating point.
• You are guaranteed to have only one unique value in the BST that is closest to the target.

Example:

Input:

 root = [4,2,5,1,3], target = 3.714286
    4
   / \
  2   5
 / \
1   3
Output:

 4