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.
两两翻转，不能借助其它的辅助空间。
java代码：
public ListNode swapPairs(ListNode head) {
if (head == null || head.next == null) return head;
ListNode node = head.next;
head.next = swapPairs(node.next);
node.next = head;
return node;
}
