diff --git a/python/543-Diameter-of-Binary-Tree.py b/python/543-Diameter-of-Binary-Tree.py
index 46a465649..78ec03116 100644
--- a/python/543-Diameter-of-Binary-Tree.py
+++ b/python/543-Diameter-of-Binary-Tree.py
@@ -5,17 +5,19 @@
 #         self.left = left
 #         self.right = right
 class Solution:
-    def diameterOfBinaryTree(self, root: TreeNode) -> int:
-        res = [0]
+    def diameterOfBinaryTree(self, root: Optional[TreeNode]) -> int:
+        res = 0
 
         def dfs(root):
+            nonlocal res
+
             if not root:
-                return -1
+                return 0
             left = dfs(root.left)
             right = dfs(root.right)
-            res[0] = max(res[0], 2 + left + right)
+            res = max(res, left + right)
 
             return 1 + max(left, right)
 
         dfs(root)
-        return res[0]
+        return res