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.

递归again？？？！！！

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