/**
 * 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) {
        if(head == NULL)
            return head;
        ListNode* p = head;
        int tmp = p->val;
        ListNode* pre = p;
        p = p->next;
        while(p)
        {
            if(p->val == tmp)
            {
                pre->next = p->next;
                p = pre;
            }
            pre = p;
            tmp = pre->val;
            p = p->next;
        }
        return head;
    }
};