#451 Swap Nodes in Pairs

题目描述：

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

Have you met this question in a real interview? 

Yes

Example

Given 1->2->3->4, you should return the list as 2->1->4->3.

Challenge 

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

题目思路：

这题继续用好用的dummy head，然后就是指针swap，需要注意的是swap过程中的顺序。

Mycode（AC = 31ms）：

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    /**     * @param head a ListNode     * @return a ListNode     */    ListNode* swapPairs(ListNode* head) {        // Write your code here        ListNode *dummy = new ListNode(0);        dummy->next = head;                ListNode *tmp = dummy, *next = dummy->next;        while (next && next->next) {            tmp->next = next->next;            next->next = tmp->next->next;            tmp->next->next = next;                        tmp = next;            next = tmp->next;        }                return dummy->next;    }};