re-solve "226. Invert Binary Tree"

Author: lancelote

src/invert_binary_tree.py

@@ -3,10 +3,11 @@
 
 class Solution:
     def invertTree(self, root: TreeNode | None) -> TreeNode | None:
-        if not root:
+        if root is None:
             return None
 
         root.left, root.right = root.right, root.left
+
         self.invertTree(root.left)
         self.invertTree(root.right)