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;
}