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.

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode* swapPairs(ListNode* head) {
12         if(!head)
13             return head;
14         ListNode* dummy=new ListNode(-1),*pre=dummy;
15         dummy->next=head;
16         while(pre->next&&pre->next->next)
17         {
18             ListNode* last=pre->next->next;
19             pre->next->next=last->next;
20             last->next=pre->next;
21             pre->next=last;
22             pre=last->next;
23         }
24         return dummy->next;
25     }
26 };