添加0102.二叉树的层序遍历递归解法Python代码

problems/0102.二叉树的层序遍历.md

@@ -88,6 +88,7 @@ public:
 python代码：
 
 ```python
+# 迭代法
 class Solution:
     def levelOrder(self, root: TreeNode) -> List[List[int]]:
         if not root:
@@ -108,7 +109,20 @@ class Solution:
 
         return out_list
 ```
-
+```python
+# 递归法
+class Solution:
+    def levelOrder(self, root: TreeNode) -> List[List[int]]:
+        res = []
+        def helper(root, depth):
+            if not root: return []
+            if len(res) == depth: res.append([]) # start the current depth
+            res[depth].append(root.val) # fulfil the current depth
+            if  root.left: helper(root.left, depth + 1) # process child nodes for the next depth
+            if  root.right: helper(root.right, depth + 1)
+        helper(root, 0)
+        return res
+```
 java: 
 
 ```Java