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.

代码如下：
public class SwapNodesInPairs {
//accepted
public ListNode swapPairs(ListNode head){
ListNode dummy = new ListNode(0);
dummy.next = head;

ListNode current = dummy;

while(current.next != null && current.next.next !=null){
ListNode first = current.next;
ListNode second = current.next.next;
first.next = second.next;
current.next = second;
current.next.next = first;
current = current.next.next;
}
return dummy.next;
}
}