Add solution 98.

Author: javadev

src/Algorithms/0098.validate-binary-search-tree/validate-binary-search-tree.php

@@ -0,0 +1,25 @@
+/**
+ * Definition for a binary tree node.
+ * class TreeNode {
+ *     public $val = null;
+ *     public $left = null;
+ *     public $right = null;
+ *     function __construct($value) { $this->val = $value; }
+ * }
+ */
+class Solution {
+
+    /**
+     * @param TreeNode $root
+     * @return Boolean
+     */
+    function isValidBST($root) {
+        return self::helper($root, null, null);
+    }
+    
+    function helper($node, $min, $max) {
+        if ($node == null) return true;
+        if (($min != null && $node->val <= $min->val) || ($max != null && $node->val >= $max->val)) return false;
+        return self::helper($node->left, $min, $node) && self::helper($node->right, $node, $max);
+    }
+}