LeetCode: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 dummy(-1);
        
        ListNode *pre=&dummy;
        ListNode *cur=head;
        ListNode *next=cur->next;
        
        while(cur!=NULL&&next!=NULL)
        {
            pre->next=next;
            cur->next=next->next;
            next->next=cur;
            
            pre=pre->next->next;
            if(cur->next!=NULL)
            {
            cur=cur->next;
            next=cur->next;
            }
            else
                break;
        }
        
        return dummy.next;
      
    }
};