Node* reverse(Node* p){
    
    Node* head = p;
    
    if(p == nullptr) //nullptr requires c++11
        return head;
    
    head = cons(car(p),nullptr);
    p = cdr(p);
    
    while(p != nullptr){
        head = cons(car(p),head);
        p = cdr(p);
    }
    
    return head;
    
}