关键：1.维持三个指针pPrePre, pPre, pCurrent（两个其实也可以）；2.使用头结点dummy简化过程；

class Solution {public:    ListNode *deleteDuplicates(ListNode *head) {        ListNode* dummy = new ListNode(-1);        dummy->next = head;        ListNode* pPrePre = dummy;        if (pPrePre == NULL) return dummy->next;        ListNode* pPre = pPrePre->next;        if (pPre == NULL) return dummy->next;        ListNode* pCur = pPre->next;        if (pCur == NULL) return dummy->next;        while (!(pPrePre == NULL || pPre == NULL || pCur == NULL) )// not end        {            bool eq = false;            while (pCur != NULL && pPre->val == pCur->val)            {                eq = true;                pPre->next = pCur->next;                pCur = pCur->next;            }            if (eq)            {                pPrePre->next = pCur;                pPre = pCur;                if (pCur != NULL)                {                    pCur = pCur->next;                }            }            else            {                pPrePre = pPre;                pPre = pCur;                if (pCur != NULL)                {                    pCur = pCur->next;                }            }        }        return dummy->next;    }};