/*
 * We should delete unused bypassed nodes otherwise
 * if you had n nodes to begin with and deleted m nodes 
 * you will still be consuming n nodes worth of memory. 
 */
 
ListNode* deleteDuplicates(ListNode* head) {
        if(!head or !head->next) { // base case
            return head;
        }

        auto curr = head, ahead = head->next; // adjacent nodes
        while(ahead) {
            if(curr->val == ahead->val) {
                auto temp = ahead;
                curr->next = ahead->next; // skip duplicate
                ahead = ahead->next;
                delete temp; // delete unused node
            } else {
                curr = curr->next;
                ahead = ahead->next;
            }
        }

        return head;
    }