LeetCode 024 Swap Nodes in Pairs 交换链表中相邻的两个节点

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.
  * struct ListNode {
  *     int val;
  *     ListNode *next;
  *     ListNode(int x) : val(x), next(NULL) {}
  * };
  */
 class Solution {
 public:
     ListNode* swapPairs(ListNode* head) {
         if(!head)
             return head;
         ListNode* dummy=new ListNode(-),*pre=dummy;
         dummy->next=head;
         while(pre->next&&pre->next->next)
         {
             ListNode* last=pre->next->next;
             pre->next->next=last->next;
             last->next=pre->next;
             pre->next=last;
             pre=last->next;
         }
         return dummy->next;
     }
 };