```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;
}
}
