You are given the `root` of a binary search tree (BST) and an integer `val`.

Find the node in the BST that the node's value equals `val` and return the subtree rooted with that node. If such a node does not exist, return `null`.

Example 1:

    Input: root = [4,2,7,1,3], val = 2
    Output: [2,1,3]

Example 2:

    Input: root = [4,2,7,1,3], val = 5
    Output: []

Constraints:

- The number of nodes in the tree is in the range `[1, 5000]`.
- `1 <= Node.val <= 107`
- `root` is a binary search tree.
- `1 <= val <= 107`