/**
 * 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) {
        auto h = new ListNode(-1);  // 虚结点，作用是避免找不到头结点，防止头结点被删除
        auto p = h;
        h -> next = head;
        auto q = h -> next;  // q结点为p结点的后一个结点

        while(q)
        {
            while(q -> next && q -> next -> val == q -> val) q = q -> next;

            if(p -> next == q)
            {
                p = q;
                q = q -> next;  // 可把if-else语句中的相同语句提出去
            }
            else
            {
                p -> next = q -> next;
                q = q -> next;
            }
        }


        return h -> next;



    }
};