24. Swap Nodes in Pairs

```java
/**
 * 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 res = head.next;
        head.next = res.next;
        res.next = head;
        while(true){
            ListNode A = head.next;
            if(A==null)break;
            ListNode B = A.next;
            if(B==null)break;
            A.next = B.next;
            B.next = A;
            head.next = B;
            head = A;
        }
        return res;
    }
}