验证二叉搜索树

<?php
Class TreeNode {
    /** @var int $val */
    public $val = 0;
    /** @var TreeNode $left */
    public $left;
    /** @var TreeNode $right */
    public $right;
    public function __construct($val) {
        $this->val = $val;
    }
}
class Solution {
    public array $res = [];
    // 是否二叉搜索树
    public function isValidBST($root) {
        return $this->valid($root, null, null);
    }
    /**
     * 递归判断是否二叉搜索树
     * 左<中<右
     * @param $root
     * @param $min
     * @param $max
     * @return bool
     */
    private function valid($root, $min, $max) {
        if (is_null($root)) return true;
        if (!is_null($min) && $root->val <= $min) return false;
        if (!is_null($max) && $root->val >= $max) return false;
        return $this->valid($root->left, $min, $root->val) && $this->valid($root->right, $root->val, $max);
    }
}
/**
 *          4
 *         / \
 *        2   6
 *       / \ /
 *      1  3 5
 */
$tree = new TreeNode(4);
$tree->left = new TreeNode(2);
$tree->left->left = new TreeNode(1);
$tree->left->right = new TreeNode(3);
$tree->right = new TreeNode(6);
$tree->right->left = new TreeNode(5);
$cls = new Solution();
$r = $cls->isValidBST($tree);
echo $r ? 1 : 0;