Update 0098.验证二叉搜索树.md

补充：**递归** - 避免初始化最小值做法

problems/0098.验证二叉搜索树.md

@@ -392,7 +392,26 @@ class Solution:
             return is_left_valid and is_right_valid
         return __isValidBST(root)
 ```
+**递归** - 避免初始化最小值做法：
+```python
+class Solution:
+    def isValidBST(self, root: TreeNode) -> bool:
+        # 规律: BST的中序遍历节点数值是从小到大. 
+        pre = None
+        def __isValidBST(root: TreeNode) -> bool: 
+            nonlocal pre
             
+            if not root: 
+                return True
+            
+            is_left_valid = __isValidBST(root.left)
+            if pre and pre.val>=root.val: return False
+            pre = root
+            is_right_valid = __isValidBST(root.right)
+            
+            return is_left_valid and is_right_valid
+        return __isValidBST(root)
+```
 ```python
 迭代-中序遍历
 class Solution: