24. 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.

两个一组，保存好前驱，交换完毕继续推进。开头也要换，所以增加一个冗余节点作为辅助。
public class Solution {
public static ListNode swapPairs(ListNode head)
{
if(head==null)
return head;

ListNode dummy=new ListNode(6);
dummy.next=head;
ListNode p1=head;
ListNode p2=head.next;
ListNode prev=dummy;

while(p2!=null)
{
p1.next=p2.next;
p2.next=p1;
prev.next=p2;
prev=p1;
p1=p1.next;
if(p1!=null)
p2=p1.next;
else {
break;
}
}

return dummy.next;
}
}

还有递归方法： https://discuss.leetcode.com/topic/4351/my-accepted-java-code-used-recursion
public class Solution {
public ListNode swapPairs(ListNode head) {
if ((head == null)||(head.next == null))
return head;
ListNode n = head.next;
head.next = swapPairs(head.next.next);
n.next = head;
return n;
}
}