LeetCode:Swap Nodes in Pairs

 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==NULL || head->next==NULL)  return head;
13        
14         ListNode dummy(-1);
15         
16         ListNode *pre=&dummy;
17         ListNode *cur=head;
18         ListNode *next=cur->next;
19         
20         while(cur!=NULL&&next!=NULL)
21         {
22             pre->next=next;
23             cur->next=next->next;
24             next->next=cur;
25             
26             pre=pre->next->next;
27             if(cur->next!=NULL)
28             {
29             cur=cur->next;
30             next=cur->next;
31             }
32             else
33                 break;
35         }
36         
37         return dummy.next;
38       
39     }
40 };