Swap Nodes in Pairs

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.

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/** * 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)            return null;        ListNode cur = head;        ListNode newHead = new ListNode(0);        ListNode end = newHead;        while(cur != null){            ListNode temp;            end.next = cur;            if(cur.next != null){                cur = cur.next;                temp = cur.next;            }else{                break;            }            end.next.next = null;            cur.next = end.next;            end.next = cur;            end = cur.next;            cur = temp;        }                return newHead.next;    }}