利用虛擬節點進行刪除結點，pre始終指向不重復的錢一個元素，一開始指向虛擬結點。如果遇到重復結點就全部刪除再與pre相連接.

/**
* 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
 
* dummyNode;
        dummyNode = new ListNode(0);
        dummyNode->next = head;
        ListNode* pre;
        pre = dummyNode;
        ListNode* cur;
        cur = head;
        if (head == NULL)
            return NULL;
        while (cur->next!=NULL)//pre指向沒重復的前一個元素,cur指向當前元素
        {
            
 
if(cur->next->val == cur->val)
            {
                if(cur->val == cur->next->val)
                {
                    int val = cur->val;
                    while (cur!=NULL&&cur->val == val)
                    {
                        ListNode* delNode;
                        delNode 
 
= cur;
                        cur = cur->next;
                        delete delNode;
                    }
                    pre->next = cur;
                    if (cur == NULL)
                        break;
                }
            }
            else
            {
                pre = cur;
                cur = cur->next;
            }
        }
        return dummyNode->next;
    }
};

leetcode82. Remove Duplicates from Sorted List II