Update 206._reverse_linked_list.md

206._reverse_linked_list.md

-###206. Reverse Linked List
+### 206. Reverse Linked List
 
 题目:
 <https://leetcode.com/problems/reverse-linked-list/>
@@ -12,7 +12,7 @@ Easy
 
 
 
-```
+```python
 class Solution(object):
     def reverseList(self, head):
         """
@@ -33,20 +33,18 @@ class Solution(object):
 递归版本，可以再消化一下.
 
 
-```
+```python
 class Solution(object):
     def reverseList(self, head):
         """
         :type head: ListNode
         :rtype: ListNode
         """
-        return self.doReverse(head, None)
-        
-        
-    def doReverse(self, head, newHead):
-        if head == None:
-            return newHead
+        return self.reverseHelper(head, None)
+    
+    def reverseHelper(self, head, new_head):
+        if not head:
+            return new_head
         nxt = head.next
-        head.next = newHead
-        return self.doReverse(nxt, head)
-```
\ No newline at end of file
+        head.next = new_head
+        return self.reverseHelper(nxt, head)```