swap nodes in pairs

Given a linked list, swap every two adjacent nodes and return its head.

For example,

Given1->2->3->4, you should return the list as2->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.

class Solution
{
public:
ListNode* swapPairs(ListNode* head)
{
if (head == NULL || head->next == NULL)
return head;
ListNode dummy(-1);
dummy.next = head;
ListNode* pre = &dummy;
int tmp = 0;
for(ListNode* before = head, *after = head->next; after;  )
{
tmp = before->val;
before->val = after->val;
after->val = tmp;
before = before->next ? before->next->next : NULL;
after = after->next ? after->next->next : NULL;
}

return dummy.next;
}
};