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.

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1. public ListNode swapPairs(ListNode head) {
2. if(head==null) return null;
3. if(head.next==null) return head;
4. ListNode temp=head.next;
5. ListNode forward=head.next.next;
6. temp.next=head;
7. head.next=swapPairs(forward);
8. return temp;
9. }