feat: 206.reverse-linked-list add Python3 implementation (azl397985856#125)

Author: ybian19

problems/206.reverse-linked-list.md

@@ -25,7 +25,7 @@ A linked list can be reversed either iteratively or recursively. Could you imple
 
 ## 代码
 
-语言支持：JS，C++
+语言支持：JS, C++, Python
 
 JavaScript Code：
 
@@ -44,19 +44,19 @@ JavaScript Code：
  * Testcase Example:  '[1,2,3,4,5]'
  *
  * Reverse a singly linked list.
- * 
+ *
  * Example:
- * 
- * 
+ *
+ *
  * Input: 1->2->3->4->5->NULL
  * Output: 5->4->3->2->1->NULL
- * 
- * 
+ *
+ *
  * Follow up:
- * 
+ *
  * A linked list can be reversed either iteratively or recursively. Could you
  * implement both?
- * 
+ *
  */
 /**
  * Definition for singly-linked list.
@@ -86,7 +86,9 @@ var reverseList = function(head) {
 };
 
 ```
+
 C++ Code：
+
 ```C++
 /**
  * Definition for singly-linked list.
@@ -112,6 +114,26 @@ public:
     }
 };
 ```
+
+Python Code:
+
+```python
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+class Solution:
+    def reverseList(self, head: ListNode) -> ListNode:
+        if not head: return None
+        prev = None
+        cur = head
+        while cur:
+            cur.next, prev, cur = prev, cur, cur.next
+        return prev
+```
+
 ## 拓展
 
 通过单链表的定义可以得知，单链表也是递归结构，因此，也可以使用递归的方式来进行reverse操作。
@@ -133,7 +155,7 @@ public:
         ListNode* tail = nullptr;
         return reverseRecursive(head, tail);
     }
-    
+
     ListNode* reverseRecursive(ListNode *head, ListNode *&tail) {
         if (head == nullptr) {
             tail = nullptr;
@@ -160,7 +182,7 @@ public:
         if (head == nullptr) return head;
         return reverseRecursive(nullptr, head, head->next);
     }
-    
+
     ListNode* reverseRecursive(ListNode *prev, ListNode *head, ListNode *next)
     {
         if (next == nullptr) return head;