add 98.py 驗證二叉樹(中序)

lky-leetcode · lky-leetcode · commit 4fdb4392c949 · 2020-05-06T01:04:02.000+08:00
diff --git a/medium/tree/98.py b/medium/tree/98.py
@@ -0,0 +1,24 @@
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+class Solution:
+    def isValidBST(self, root: TreeNode) -> bool:
+        if not root:
+            return True
+        self.ans = True
+        self.prev = float('-inf')
+        def inorder(root):
+            if not root:
+                return
+            inorder(root.left)
+            if self.prev >= root.val:
+                self.ans = False
+            self.prev = root.val
+            inorder(root.right)
+
+        inorder(root)
+        return self.ans
