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.

public class Solution{public ListNode swapPairs(ListNode head) {if (head == null || head.next == null)return head;ListNode second = head.next;ListNode third = second.next;second.next = head;head.next = swapPairs(third);return second;}}//从题目的意思上看，就知道这题目有很多重复性的动作，可以通过递归来解决，不难~