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class Solution {    public:    ListNode* swapPairs(ListNode* head) {        if(!head||!head->next)            return head;        ListNode *headptr=new ListNode(0);        headptr->next=head;        ListNode* pre;        ListNode *before;        ListNode *after;        ListNode *nextpair;        pre=headptr;        before=head;        after=head->next;        while(after!=NULL&&before!=NULL)        {            nextpair=after->next;            after->next=before;            pre->next=after;            pre=before;            if(nextpair!=NULL)            {                before=nextpair;                after=nextpair->next;                if(!after)                {                    pre->next=before;                }            }            else {                before->next=NULL;                break;            }        }        return headptr->next;    }};

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