Closest Binary Search Tree Value

Given the root of a binary search tree and a target value, return the value in the BST that is closest to the target. If there are multiple answers, return the smallest such value. A binary search tree satisfies these constraints:

• The left subtree of every node contains only nodes with keys less than the node's key.
• The right subtree of every node contains only nodes with keys greater than the node's key.
• Both the left and right subtrees are also BSTs.