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.
Solutions：
三个指针，开始的时候new一个节点，其next指向head，结束的时候delete掉。

class Solution {
public:
ListNode* swapPairs(ListNode* head) {
if(head==NULL) {
return head;
}
ListNode *prev=NULL;
if(head->next==NULL)
return head;
ListNode *ret=new ListNode(0);
prev=ret;
prev->next=head;
ListNode *p1=head, *p2=p1->next;
while(1) {
prev->next=p2;
p1->next=p2->next;
p2->next=p1;
prev=p1;
if(p1->next==NULL)
break;
p1=prev->next;
if(p1->next==NULL)
break;
p2=p1->next;
}
prev=ret;
ret=ret->next;
delete prev;
return ret;
}
};