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;    }};