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.

方法

更改指针，就可以。

	    public ListNode swapPairs(ListNode head) {
	       if(head == null || head.next == null){
	           return head;
	       }else{
	           ListNode first = null;
	           ListNode second = null;
	           ListNode suc = null;
	           ListNode pre = null;           
	           first = head;
	           head = null;
	           while(first.next != null){
	               second = first.next;
	               suc = second.next;
	               
	               if(pre == null){
	                   head = second;
	                   second.next = first;
	                   first.next = suc;
	                   pre = first;
	               }else{
	                   pre.next = second;
	                   second.next = first;
	                   first.next = suc;
	                   pre = first;
	               }
	               first = suc;
	               if(first == null){
	                   break;
	               }
	           }        
	           return head;
	       }
	    }