删除链表中重复的节点

在一个排序的链表中，存在重复的节点，请删除该链表中重复的节点，重复的节点不保留。

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/**  * Definition for singly-linked list.  * struct ListNode {  *     int val;  *     ListNode *next;  *     ListNode(int x) : val(x), next(NULL) {}  * };  */ class Solution { public:     ListNode* deleteDuplication(ListNode* head) {         if (!head) return NULL;         auto dummy = new ListNode(-1);         dummy->next = head;         auto p = dummy;                   while (p->next) {             auto q = p->next->next;             while (q && q->val == p->next->val) q = q->next;                           if (q == p->next->next) p = p->next;             else p->next = q;         }                   return dummy->next;     } };