LeetCode OJ:Swap Nodes in Pairs

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* reverse(ListNode *pre, ListNode *next){  
        ListNode *last = pre->next;  
        ListNode *cur = last->next;  
        while(cur != next){  
            last->next = cur->next;  
            cur->next = pre->next;  
            pre->next = cur;  
            cur = last->next;  
        }  
        return last;  
    }  
    ListNode *swapPairs(ListNode *head) {
        ListNode *nh = new ListNode(0);  
        nh->next = head;  
        ListNode *pre = nh;  
        int count = 0;  
        while(head){  
            count++;  
            if(count % 2 ==0){  
                pre = reverse(pre, head->next);  
                head = pre->next;  
            }else {  
                    head = head->next;  
            }  
        }  
        return nh->next; 
    }
};