83. remove duplicates from sorted list

class Solution {
public:
    ListNode* (ListNode* head) {
        if (head != NULL) {
            ListNode *p = head;
            while (p->next != NULL) {
                int val = p->val;
                if (val == p->next->val) {
                    ListNode *tmp = p->next;
                    p->next = p->next->next;
                    free(tmp);
                } else {
                    p = p->next;
                }
            }
        }
        return head;
    }
};