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 || head->next==NULL)
            return head;
        ListNode* p1=head;
        ListNode* p2=head->next;
        head=p2;
        ListNode* pre=NULL;
        while(p1!=NULL && p2!=NULL)
        {
            if(pre!=NULL)
                pre->next=p2;
            p1->next=p2->next;
            p2->next=p1;
            
            pre=p1;
            
            p1=p1->next;
            if(p1!=NULL)
                p2=p1->next;
        }
        return head;
    }
};