LeetCode: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.

思路：添加一个空的头指针，每两个结点一组，先链接后一个结点，再链接前一个结点（注意链接完后将该结点的next指针置为空）。

 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)
13             return head;
14         ListNode *p=head,*q=p->next,*r;
15         r->next=NULL;
16         head=r;
17         while(p!=NULL)
18         {
19             if(q!=NULL)
20             {
21                 head->next=q;
22                 q=q->next;
23                 head=head->next;
24             }
25             head->next=p;
26             head=head->next;
27             head->next=NULL;
28             p=q;
29             if(p!=NULL&&p->next!=NULL)
30                 q=p->next;
31         }
32         return r->next;
33     }
34 };