void Polynomial::insertTerm(double coeff, int exp) {
    if (coeff != 0) {
        if (head == NULL || exp > head -> exp){
            insertNode(NULL, coeff, exp);
            return;
        }
        Node *temp = head;
        while (temp -> link != NULL && temp -> link -> exp > exp)
            temp = temp -> link;
        if (temp -> link == NULL){
            if (temp -> exp == exp){
                temp -> coeff += coeff;
                if (temp -> coeff == 0)
                    deleteNode(NULL, temp);
            }
            else
                insertNode(temp, coeff, exp);
        }
        else {
            if (exp == temp -> link -> exp){
                temp -> link -> coeff += coeff;
                if (temp -> link -> coeff == 0)
                    deleteNode(temp, temp -> link);
            }
            else
                insertNode(temp, coeff, exp);
        }
    }
    return;
}