24. 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.

 1 class Solution {
 2     public ListNode swapPairs(ListNode head) {
 3         ListNode fakehead = new ListNode(0);
 4         fakehead.next = head;
 5         ListNode pre = fakehead;
 6         ListNode cur = head;
 7         while(cur !=null && cur.next != null){
 8             ListNode next = cur.next;
 9             pre.next = next;
10             cur.next = next.next;
11             next.next = cur;
12             pre = cur;
13             cur = pre.next;
14         }
15         return fakehead.next;
16     }
17 }