# Problem 4: Binary Decision Tree (Search in a Binary Search Tree)

## Problem Statement
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`.

## Input Format
- The root of a BST.
- An integer `val`.

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