24. Swap Nodes in Pairs
```java
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode() {}
* ListNode(int val) { this.val = val; }
* ListNode(int val, ListNode next) { this.val = val; this.next = next; }
* }
*/
class Solution {
public ListNode swapPairs(ListNode head) {
if (head == null){
return head;
}
ListNode n = head.next;
head.next = swapPairs(head.next.next);
n.next = head;
return n;
}
}
```
