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.

C++:
01 ListNode *swapPairs(ListNode *head) {
02         if(head==NULL)
03             return NULL;
04         if(head->next==NULL)
05             return head;
06         ListNode* hd=head->next;
07         ListNode* tmp=hd->next;
08         hd->next=head;
09         head->next=swapPairs(tmp);
10         return hd;
11 }
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