Swap Nodes in Pairs

//Swap Nodes in Pairs Total Accepted: 43260 Total Submissions: 133314 My Submissions Question Solution 
//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.
public class SwapNodesinPairs {


public static void main(String[] args) {
// TODO 自动生成的方法存根


}
public ListNode swapPairs(ListNode head) {
  if(head==null||head.next==null)
    return head;
   ListNode prehead=new ListNode(0);
   ListNode p=prehead;
   ListNode one=head;
   ListNode two=head.next;
  
   while(true){
    ListNode three=two.next;
    p.next=two;
    two.next=one;
    one.next=three;
    p=p.next.next;
    //接下来没有了
    if(p.next==null)
    break;
    one=p.next;
    //接下来有一个
    if(one.next==null)
    break;
    two=one.next;
   
   }
   //是使用prehead.next
   return prehead.next;
}


}