struct ListNode* deleteDuplication(struct ListNode* head) {
    if(!head)   return;
    struct ListNode * dummy = (struct ListNode*) malloc(sizeof(struct ListNode));
    dummy->next = head;
    struct ListNode * pre = dummy;
    struct ListNode * cur = head;
    while(cur)
    {
        while(cur && pre->next->val==cur->val)    cur = cur->next;
        if (pre->next->next == cur) pre = pre->next;    //如果下一段是连续的
        else pre->next = cur;   //如果下一段是之后的一段,直接接上

    }
    return dummy->next;
}