Update 34. 二叉树的层序遍历.md

Author: jasoncao11

34. 二叉树的层序遍历.md

@@ -10,6 +10,8 @@
 ```
 class Solution:
     def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+        if not root:
+            return None
         #队列+BFS
         queue = [root]
         res = []
@@ -20,12 +22,12 @@ class Solution:
             #遍历该层所有节点
             for i in range(n):
                 node = queue.pop(0)
-                if node:
+                level.append(node.val)
+                if node.left:
                     queue.append(node.left)
-                    queue.append(node.right)
-                    level.append(node.val)
-            if level:
-                res.append(level)
+                if node.right:
+                    level.append(node.right)
+            res.append(level)
         return res
 
-```
+```