LeetCode解题报告--Swap Nodes in Pairs

题目：

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.
原题：https://leetcode.com/problems/swap-nodes-in-pairs/

分析：题意要求将给定的链表，相邻的结点交换位置，并返回变化后的链表。
基本思路利用指针移位法，注意考虑特殊情况，如链表为空，链表长度为0，1，2等。
如下图是思路的示意：

不同颜色表示不同次的循环过程。

java代码 Accepted:

/** * 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 || head.next == null) {            return head;        }        ListNode p;        ListNode q;        ListNode k;        p = head;        q = p.next;        k = q.next;        head = head.next;        while (q != null) {            q.next = p;            if(k != null && k.next != null){                p.next = k.next;                p = k;                q = k.next;                k = k.next.next;            }else{                p.next = k;                q = null;                k = null;            }            /*p = k;            if(k != null){                q = k.next;            }else{                q = null;            }            if(k != null && k.next != null){                k = k.next.next;            }else{                k = null;            }*/        }        return head;    }}