[Search in a Binary Search Tree]
https://leetcode.com/problems/search-in-a-binary-search-tree/description/
class Solution {
public TreeNode searchBST(TreeNode root, int val) {
if (root == null || root.val == val) return root;
if (val < root.val) return searchBST(root.left, val);
else return searchBST(root.right, val);
}
}