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.

题目解答：成对的调转链表中的节点。使用指针p和q表示当前待交换的两个节点。o代表了前一个节点。r表示后一个节点。

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) {

        ListNode *Head = new ListNode(-1);

        Head -> next = head;

        ListNode *o = Head;

        ListNode *p = Head -> next;

        while(p != NULL)

        {

            ListNode *q = p -> next;

            if(q != NULL)

            {

                ListNode *r = q -> next;

                o -> next = q;

                q -> next = p;

                p -> next = r;

                o = o -> next -> next;

            }

            p = p -> next;

        }

        p = Head -> next;

        delete Head;

        return p;

    }

};