leetcode_swap_nodes_in_pair

//很简单的一道题

/**
 * 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 *p,*ptr,*q,*t;
        t=p=ptr=q=head;
        while(ptr!=NULL)
        {
            if(ptr->next==NULL)
               break;
            p=ptr;
            q=ptr->next;
            p->next=q->next;
            q->next=p;   //注意这里ptr跟p是一个指向，所以p改变后，ptr也会改变
            if(t==head)
               head=q;
            else
               t->next=q;
               
            t=ptr;
            if(ptr->next==NULL)
               break;
            ptr=ptr->next;
        }
        return head;
    }
};