Merge pull request #2553 from raydemo1/master

Update 0102.二叉树的层序遍历.md Python层序遍历递归法更简洁

Author: youngyangyang04

problems/0102.二叉树的层序遍历.md

@@ -201,7 +201,7 @@ class Solution:
         return result
 ```
 ```python
-# 递归法
+#递归法
 # Definition for a binary tree node.
 # class TreeNode:
 #     def __init__(self, val=0, left=None, right=None):
@@ -210,18 +210,24 @@ class Solution:
 #         self.right = right
 class Solution:
     def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+        if not root:
+            return []
+
         levels = []
-        self.helper(root, 0, levels)
+
+        def traverse(node, level):
+            if not node:
+                return
+
+            if len(levels) == level:
+                levels.append([])
+
+            levels[level].append(node.val)
+            traverse(node.left, level + 1)
+            traverse(node.right, level + 1)
+
+        traverse(root, 0)
         return levels
-    
-    def helper(self, node, level, levels):
-        if not node:
-            return
-        if len(levels) == level:
-            levels.append([])
-        levels[level].append(node.val)
-        self.helper(node.left, level + 1, levels)
-        self.helper(node.right, level + 1, levels)
 
 ```