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.

第一遍：

 1 public class Solution {

 2     public ListNode swapPairs(ListNode head) {

 3         // Note: The Solution object is instantiated only once and is reused by each test case.

 4         if(head == null || head.next == null) return head;

 5         ListNode header = new ListNode(-1), first = head, second = head.next, cur = header;

 6         header. next = head;

 7         while(second != null){

 8             first.next = second.next;

 9             second.next = first;

10             cur.next = second;

11             cur = first;

12             first = cur.next;

13             if(first != null)

14                 second = first.next;

15             else

16                 second = null;

17         }

18         return header.next;

19     }

20 }

第三遍：

 1 public class Solution {

 2     public ListNode swapPairs(ListNode head) {

 3         ListNode header = new ListNode(-1);

 4         header.next = head;

 5         ListNode cur = header;

 6         while(cur.next != null && cur.next.next != null){

 7             ListNode first = cur.next.next, second = cur.next;

 8             second.next = first.next;

 9             cur.next = first;

10             first.next = second;

11             cur = second;

12         }

13         return header.next;

14     }

15 }