Given a linked list, swap every two adjacent nodes and return its head.Example:Given 1->2->3->4, you should return the list as 2->1->4->3.Note:Your algorithm should use only constant extra space.You may not modify the values in the list's nodes, only nodes itself may be changed.

class Solution {    public ListNode swapPairs(ListNode head) {        if(head == null) return null;        ListNode dummy = new ListNode(-1);        dummy.next = head;        ListNode pre = dummy;        while(pre.next != null && pre.next.next != null){            ListNode tmp = pre.next.next;            pre.next.next = tmp.next;            tmp.next = pre.next;            pre.next = tmp;            pre = pre.next.next;        }        return dummy.next;    }}