LeetCode 129 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.

分析：

这跟 Reverse Node in k-Groups 的思路是一样的，不过这里的k变成了2.

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { *         val = x; *         next = null; *     } * } */public class Solution {    public ListNode swapPairs(ListNode head) {        if(head==null || head.next==null)             return head;        ListNode dummy = new ListNode(0);        dummy.next = head;        ListNode pre = dummy;        while(pre.next != null && pre.next.next != null){            ListNode first = pre.next;            ListNode second = pre.next.next;            //交换            first.next = second.next;            second.next = first;            pre.next = second;            //            pre = first;        }        return dummy.next;    }}