> For the complete documentation index, see [llms.txt](https://liuyang89116.gitbook.io/my-leetcode-book/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://liuyang89116.gitbook.io/my-leetcode-book/chapter_4_linked_list/reverse-linked-list/problem_24_swap_nodes_in_pairs.md).

# Problem 24: Swap Nodes in Pairs

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

## 思路

* 考察的主要就是两根指针的移动，和上一题很像

  ![](https://1241747088-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Lpv9LvBSlFaukf_ALqh%2F-Lpv9NPw3Ji1X5Vk8CES%2F-Lpv9vqr3buGR_tX8l5M%2FswapNodes.png?generation=1569729532549534\&alt=media)

```java
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;

        head = dummy;
        while (head.next != null && head.next.next != null) {
            ListNode n1 = head.next;
            ListNode n2 = head.next.next;

            // swap
            head.next = n2;
            n1.next = n2.next;
            n2.next = n1;

            head = n1;
        }

        return dummy.next;
    }
}
```

## 易错点

1. dummy node 必须要存在，因为涉及到交换 node 的时候，“头”就乱了。
2. while 的循环条件，是 head.next.next != null； 因为要涉及两根指针 n1 和 n2
3. 交换完指针记得 update head 指针

```java
// swap
head.next = n2;
n1.next = n2.next;
n2.next = n1;

head = n1;
```