Update 0098.验证二叉搜索树.md

将上一版本的代码改成跟题解思路相同.

problems/0098.验证二叉搜索树.md

@@ -372,18 +372,30 @@ class Solution:
         return res
 ```
 
-**递归** - 
+**递归** - 标准做法
 
 ```python
 class Solution:
     def isValidBST(self, root: TreeNode) -> bool:
-        def isBST(root, min_val, max_val):
-            if not root: return True
-            if root.val >= max_val or root.val <= min_val:
+        # 规律: BST的中序遍历节点数值是从小到大. 
+        cur_max = -float("INF")
+        def __isValidBST(root: TreeNode) -> bool: 
+            nonlocal cur_max
+            
+            if not root: 
+                return True
+            
+            is_left_valid = __isValidBST(root.left)
+            if cur_max < root.val: 
+                cur_max = root.val
+            else: 
                 return False
-            return isBST(root.left, min_val, root.val) and isBST(root.right, root.val, max_val)
-        return isBST(root, float("-inf"), float("inf"))
-
+            is_right_valid = __isValidBST(root.right)
+            
+            return is_left_valid and is_right_valid
+        return __isValidBST(root)
+```
+```
 # 迭代-中序遍历
 class Solution:
     def isValidBST(self, root: TreeNode) -> bool: