LeetCode: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.

思路：添加一个空的头指针，每两个结点一组，先链接后一个结点，再链接前一个结点（注意链接完后将该结点的next指针置为空）。

/**
 * 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) {
        if(head==NULL||head->next==NULL)
            return head;
        ListNode *p=head,*q=p->next,*r;
        r->next=NULL;
        head=r;
        while(p!=NULL)
        {
            if(q!=NULL)
            {
                head->next=q;
                q=q->next;
                head=head->next;
            }
            head->next=p;
            head=head->next;
            head->next=NULL;
            p=q;
            if(p!=NULL&&p->next!=NULL)
                q=p->next;
        }
        return r->next;
    }
};