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;    }};