Given a linked list, swap every two adjacent nodes and return its head.

For example,

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

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { *         val = x; *         next = null; *     } * } */public class Solution {    public ListNode swapPairs(ListNode head) {        if(head==null||head.next==null)            return head;        ListNode faked=new ListNode(0);        faked.next=head;        ListNode pre=faked;        ListNode cur=head;        while(cur!=null&&cur.next!=null){            pre.next=cur.next;            cur.next=pre.next.next;            pre.next.next=cur;            pre=cur;            cur=pre.next;        }        return faked.next;    }}

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