[LeetCode]Remove Duplicates from Sorted List II

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

For example,
Given 1->2->3->3->4->4->5, return 1->2->5.
Given 1->1->1->2->3, return 2->3.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *deleteDuplicates(ListNode *head) {
        ListNode *p,*q,*r; //前中后
        ListNode *newhead=NULL,*n;
        if(head==NULL||head->next==NULL) return head; //空链表，单结点直接返回
        p=head;
        q=p->next;
        r=q->next;
        if(head->val!=head->next->val) newhead=n=head; //若头结点不需要去掉
        if(r==NULL) //两个结点
        {
            if(p->val==q->val) return NULL; 
            else return head;
        }
        while(r)
        {
            if(p->val!=q->val&&q->val!=r->val) //前后不同，新链表加入q
            {
                if(newhead==NULL)
                {
                    newhead=n=q;
                }
                else 
                {
                    n->next=q;
                    n=q;
                }
            }
            p=p->next;
            q=q->next;
            r=r->next;
        }
        //r==NULL
        if(p->val!=q->val) //考虑无头结点情况
        {
            if(newhead==NULL)
            {
                newhead=q;
                newhead->next=NULL;
                return newhead;
            }
            else n->next=q;
        }
        else 
        {
            if(newhead==NULL) return newhead;
            else 
            {
                n->next=q;
                n->next=NULL;
            }
        }
        return newhead;
    }
};