/**
 * 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* start = new ListNode(0);
        start->next = head;

        ListNode* cur = head;
        ListNode* prev = start;
        while(cur){
            while(cur->next && cur->next->val == prev->next->val)
                cur = cur->next;
            if(prev->next == cur)
                prev = cur;
            else
                prev->next = cur->next; 
            cur = cur->next;
        }
        return start->next;
    }
};