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🐱(link): 24. 两两交换链表中的节点

补充递归题解

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docs/data-structure/linked_list
- README.md

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`‎docs/data-structure/linked_list/README.md‎`

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Original file line number	Diff line number	Diff line change
`@@ -166,14 +166,12 @@ class Solution(object):`
`166`	`166`
`167`	`167`	`## 24. 两两交换链表中的节点`
`168`	`168`
`169`		`-### 思路`
	`169`	`+### 解一:迭代`
`170`	`170`
`171`	`171`	- 为方便统一化创建空节点 `cur`
`172`	`172`	`- 交换呗,没啥好说`
`173`	`173`	`- 注意返回头节点`
`174`	`174`
`175`		`-### Python`
`176`		`-`
`177`	`175`	```python
`178`	`176`	`class Solution(object):`
`179`	`177`	`def swapPairs(self, head):`
`@@ -195,6 +193,56 @@ class Solution(object):`
`195`	`193`	`return first.next`
`196`	`194`	```
`197`	`195`
	`196`	`+### 解二:递归`
	`197`	`+`
	`198`	+```python
	`199`	`+# Definition for singly-linked list.`
	`200`	`+# class ListNode:`
	`201`	`+# def __init__(self, x):`
	`202`	`+# self.val = x`
	`203`	`+# self.next = None`
	`204`	`+`
	`205`	`+class Solution:`
	`206`	`+ def swapPairs(self, head: ListNode) -> ListNode:`
	`207`	`+ def helper(node, pre):`
	`208`	`+ if node is None or node.next is None:`
	`209`	`+ return`
	`210`	`+ next_node = node.next`
	`211`	`+ # 指针交换`
	`212`	`+ node.next = next_node.next`
	`213`	`+ next_node.next = node`
	`214`	`+ pre.next = next_node`
	`215`	`+ helper(node.next, node)`
	`216`	`+`
	`217`	`+ ans = ListNode(0)`
	`218`	`+ ans.next = head`
	`219`	`+ helper(head, ans)`
	`220`	`+ return ans.next`
	`221`	+```
	`222`	`+`
	`223`	`+优雅递归:`
	`224`	`+`
	`225`	+```python
	`226`	`+# Definition for singly-linked list.`
	`227`	`+# class ListNode:`
	`228`	`+# def __init__(self, x):`
	`229`	`+# self.val = x`
	`230`	`+# self.next = None`
	`231`	`+`
	`232`	`+class Solution:`
	`233`	`+ def swapPairs(self, head: ListNode) -> ListNode:`
	`234`	`+ if head is None or head.next is None:`
	`235`	`+ return head`
	`236`	`+`
	`237`	`+ first_node = head`
	`238`	`+ second_node = head.next`
	`239`	`+`
	`240`	`+ first_node.next = self.swapPairs(second_node.next)`
	`241`	`+ second_node.next = first_node`
	`242`	`+`
	`243`	`+ return second_node`
	`244`	+```
	`245`	`+`
`198`	`246`	`## 25. K 个一组翻转链表`
`199`	`247`
`200`	`248`	`[原题链接](https://leetcode-cn.com/problems/reverse-nodes-in-k-group/)`

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