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.

/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* swapPairs(ListNode* head)
{
if(head==NULL)
return NULL;
if(head->next==NULL)
return head;
ListNode *p1=head;
ListNode *p2=head->next;
p1->next=p2->next;
p2->next=p1;
ListNode *temp,*pre=p1;
for(ListNode *cur=p1->next;cur!=NULL;cur=cur->next){
temp=cur->next;
if(temp!=NULL){
cur->next=temp->next;
temp->next=cur;
pre->next=temp;
pre=cur;
}
}
return p2;
}
};