24. 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.

 含义：两两交换相邻的节点

    public ListNode swapPairs(ListNode head) {
        if (head  == null || head.next == null) return head;
        ListNode p = head.next;
        head.next = swapPairs(p.next);
        p.next = head;
        return p; 
    }