LeetCode-LinkedList-24. Swap Nodes in Pairs

问题：https://leetcode.com/problems/swap-nodes-in-pairs/
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.
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分析：依次两个节点一组，交换它们两个的值。
代码：

/** * 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) {        int first;        ListNode *newhead=head;        while(head != NULL && head->next != NULL){            first=head->val;            head->val=head->next->val;            head->next->val=first;            head=head->next->next;        }        return newhead;    }};