Remove Duplicates from Sorted List

/**
 * 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 NULL;

            ListNode dummy(-1);
            dummy.next = head;
            ListNode * p = head;
            p = p->next;

            while(p)
            {
                #if 0
                cout << "p->val\t" << p->val << endl;
                cout << "head->val\t" << head->val << endl;
                #endif
                if(p->val != head->val)
                {   
                    head->next = p;
                    head = p;
                }   
                p = p->next;
                // to break old link
                head->next = NULL;
            }   
            return dummy.next;
        }   
};