Leetcode题目：Swap Nodes in Pairs

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;
    }
};