# \[206]\[简单]\[递归]\[双指针] 反转链表

## 题目描述

[206. 反转链表](https://leetcode-cn.com/problems/reverse-linked-list/) [剑指 Offer 24. 反转链表](https://leetcode-cn.com/problems/fan-zhuan-lian-biao-lcof/)

反转一个单链表。

示例:

```
输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL
```

进阶: 你可以迭代或递归地反转链表。你能否用两种方法解决这道题？

## 解题思路

### 递归

```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 head is None:
            return None
        new_head = None
        def dfs(node):
            if node.next is None:
                nonlocal new_head
                new_head = node
                return node

            new_node = dfs(node.next)
            node.next = None
            new_node.next = node
            return node

        dfs(head)
        return new_head
```

### 迭代

```python
class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        slow, fast = None, head
        while fast is not None:
            new_fast = fast.next
            fast.next = slow
            slow = fast
            fast = new_fast
        return slow
```


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