LC82 Remove Duplicates from Sorted List II

class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
        if(head==NULL)
            return NULL;
        if(head->next==NULL)
            return head;
        ListNode* nhead=new ListNode(-9999);
        ListNode* tail=nhead;
        while(head!=NULL)
        {
            if(head->next!=NULL)
            {
                if(head->val!=head->next->val)   
                {
                    tail->next=head;
                    tail=tail->next;
                    head=head->next;
                    tail->next=NULL;
                }
                else if(head->val==head->next->val)
                {
                    ListNode* tmp;
                    while(head->next!=NULL&&head->val==head->next->val)
                    {
                        tmp=head->next;
                        head->next=tmp->next;
                        delete tmp;
                    }
                    tmp=head;
                    head=head->next;
                    delete tmp;
                }
            }
            else if(head->next==NULL)
            {
                tail->next=head;
                break;
            }
        }
        return nhead->next;
    }
};