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.

兩兩翻轉，不能借助其它的輔助空間。

java代碼：

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;
    }