[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(-1);
        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;
    }
};