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

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

        ListNode pre = newHead;

        while(pre.next!=null && pre.next.next!=null){
            ListNode first = pre.next;
            pre.next = first.next;
            first.next = first.next.next;
            pre.next.next = first;
            pre = first;
        }

        return newHead.next;
    }
}