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 pre1=ListNode(0);
        ListNode *pre=&pre1;
        pre->next=head;
        ListNode *first=head;
        ListNode *last=head->next; 
        ListNode *tmp=head->next;
        while(first!=NULL&&last!=NULL){
        first->next=last->next;
        last->next=first;
        pre->next=last;
        
        first=first->next;
        if(first!=NULL)
        {
        last=first->next;
        pre=pre->next->next;
        }
        }
        return tmp;
        
        
    }
};