【链表&交换邻节点】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.
链表操作，注意头结点，交换的pre节点是否为null
public class Solution {
public ListNode swapPairs(ListNode head) {
int k = 0;
ListNode p = head, pre=null;
while(p != null){
ListNode r = null;
k++;
if(k%2 == 0){//r指向要交换的第一节点，p指向要交换的第二个节点，pre指向第一个节点的前一个节点
if(pre == null){
r = head;
r.next = p.next;
p.next = r;
head = p;
pre = r;
}
else{
r = pre.next;
r.next = p.next;
p.next = r;
pre.next = p;
pre = r;
}
p = r.next;
}
else p = p.next;
}
return head;
}
}