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;
    }
}