Update 0701.二叉搜索树中的插入操作.md

0701.二叉搜索树中的插入操作.md Python3 **递归法** - 无返回值 有注释 不用Helper function

problems/0701.二叉搜索树中的插入操作.md

@@ -330,6 +330,27 @@ class Solution:
         return root
 ```
 
+**递归法** - 无返回值 有注释 不用Helper function
+```
+class Solution:
+    last = None
+    def insertIntoBST(self, root: Optional[TreeNode], val: int) -> Optional[TreeNode]:
+        if not root:    # for root==None
+            return TreeNode(val)
+        if root.val<val:
+            if root.right==None:    # find the parent
+                root.right = TreeNode(val)
+            else:   # not found, keep searching
+                self.insertIntoBST(root.right, val)
+        if root.val>val:
+            if root.left==None: # found the parent
+                root.left = TreeNode(val)
+            else:   # not found, keep searching
+                self.insertIntoBST(root.left, val)
+        # return the final tree
+        return root
+```
+
 **迭代法**  
 与无返回值的递归函数的思路大体一致 
 ```python