# 24.Swap-Nodes-in-Pairs

## ้ข็ฎๅฐๅ

https://leetcode.com/problems/swap-nodes-in-pairs/

## ้ข็ฎๆ่ฟฐ

``````Given a linked list, swap every two adjacent nodes and return its head.

You may not modify the values in the list's nodes, only nodes itself may be changed.

Example:
Given 1->2->3->4, you should return the list as 2->1->4->3.``````

## ไปฃ็ 

### Approach #1 Recursive Approach

``````/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
class Solution {
public ListNode swapPairs(ListNode head) {
if (head == null || head.next == null) {
return head;
}

ListNode firstNode = head;
ListNode secondNode = head.next;

firstNode.next = swapPairs(secondNode.next);
secondNode.next = firstNode;

return secondNode;
}
}``````

### Approach #2 Iterative

``````class Solution {
public ListNode swapPairs(ListNode head) {
ListNode dummy = new ListNode(-1);

dummy.next = head;
ListNode prevNode = dummy;
while (head != null && head.next != null) {
ListNode firstNode = head;
ListNode secondNode = head.next;

prevNode.next = secondNode;
firstNode.next = secondNode.next;
secondNode.next = firstNode;

prevNode = fristNode;
head = firstNode.next;
}

return dummy.next;
}
}``````

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