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.

/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode swapPairs(ListNode head) {
if(head == null)
return null;
ListNode next = head.next;
if(next == null)
return head;
ListNode nextNext = next.next;
ListNode newNext = swapPairs(nextNext);
next.next = head;
head.next = newNext;
return next;
}
}

注意，要先递归反转后面的list，再反转当前的list