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==NULL || head->next==NULL) return head;
13         ListNode *pre=head;
14         ListNode *cur=head;
15         ListNode *tmp;
16         while(cur && cur->next)
17         {
18             tmp = cur->next;
19             cur->next = cur->next->next;
20             tmp->next = cur;
21             if(pre==head)
22                 head = tmp;
23             else
24                 pre->next = tmp;
25             pre=cur;
26             cur = cur->next;
27         }
28         return head;
29     }
30 };