Given the root node of a binary search tree (BST) and a value. You need to find the node in the BST that the node's value equals the given value. Return the subtree rooted with that node. If such node doesn't exist, you should return NULL.
In the example above, if we want to search the value5, since there is no node with value5, we should returnNULL.

Note that an empty tree is represented byNULL, therefore you would see the expected output (serialized tree format) as [], notnull.

这个题也比较简单，按照BST性质来就可以

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode searchBST(TreeNode root, int val) {
        if(root == null)
            return null;

        TreeNode p = root;
        while(p != null) {
            if(p.val == val) {
                return p;
            } else if(p.val > val) {
                p = p.left;
            } else {
                p = p.right;
            }
        }

        return null;
    }
}