Given the root of a binary search tree and a target value, return the value in the BST that is closest to the target.
Example 1:
Input: root = [4,2,5,1,3], target = 3.714286 Output: 4
Example 2:
Input: root = [1], target = 4.428571 Output: 1
Constraints:
- The number of nodes in the tree is in the range
[1, 104]. 0 <= Node.val <= 109-109 <= target <= 109