[Leetcode] Swap nodes in pairs 成对交换结点

Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given1->2->3->4, you should return the list as2->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.

题意：成对交换结点。

思路：这题感觉是reverse nodes in k grops的是一个特殊情况。这题更简单一些，只要保证当前结点cur和其后继存在就可以交换。改变表头，所以要new一个。代码如下：

 /**
  * Definition for singly-linked list.
  * struct ListNode {
  *     int val;
  *     ListNode *next;
  *     ListNode(int x) : val(x), next(NULL) {}
  * };
  */
 class Solution {
 public:
     ListNode *swapPairs(ListNode *head)
     {
         ListNode *nList=new ListNode(-);
         nList->next=head;
         ListNode *pre=nList;
         ListNode *cur=head;

         while(cur&&cur->next)
         {
             ListNode *temp=cur->next;
             cur->next=temp->next;
             temp->next=pre->next;
             pre->next=temp;
             pre=cur;
             cur=cur->next;
         } 

         return nList->next;
     }
 };