fixed condition to follow the algorithm design

Author: kousukekikuchi1984

python/0098-validate-binary-search-tree.py

@@ -9,7 +9,7 @@ def isValidBST(self, root: TreeNode) -> bool:
         def valid(node, left, right):
             if not node:
                 return True
-            if not (node.val < right and node.val > left):
+            if not (left < node.val < right):
                 return False
 
             return valid(node.left, left, node.val) and valid(