leetcode 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.

解题思路：

利用三个指针实现，p1为每对pair里面的前一个数，p2为后一个数。

代码：

public ListNode swapPairs(ListNode head) {        if (head == null || head.next == null)            return head;        ListNode prev = head;        ListNode p1 = head;        ListNode p2;        while (p1 != null && p1.next != null) {            p2 = p1.next;            p1.next = p2.next;            p2.next = p1;            if (prev == head)                head = p2;            else                prev.next = p2;            prev = p1;            p1 = p1.next;        }        return head;    }