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) {
        ListNode dummy(0);
        dummy.next = head;
        ListNode *next(NULL), *pre(&dummy);
        
        while(head){
            if(head->next == NULL)
              return dummy.next;
            next = head->next->next;
            //swap adjacent two nodes
            head->next->next = head;
            pre->next = head->next;
            head->next = next;
            
            pre = pre->next->next;
            head = next;
        }
        return dummy.next;
    }
};