AddTwoNumber using linked list

gistfile1.txt

void append(ListNode **head_ref, int new_data)
{

    ListNode *new_node = new ListNode();

    ListNode *last = *head_ref;

    new_node->val = new_data;
    new_node->next = NULL;

    if (*head_ref == NULL)
    {
        *head_ref = new_node;
        return;
    }

    while (last->next != NULL)
    {
        last = last->next;
    }

    last->next = new_node;
    return;
}
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        struct ListNode *output = NULL;
    int reminder = 0;
    int result = 0;
    int sum = 0;

    do
    {
        sum = l1->val + l2->val + reminder;
        result = sum % 10;
        append(&output, result);
        reminder = sum / 10;

        l1 = l1->next;
        l2 = l2->next;

    } while (l1 != NULL && l2 != NULL);

    while (l1 != NULL)
    {
        sum = l1->val + reminder;
        result = sum % 10;
        append(&output, result);
        reminder = sum / 10;

        l1 = l1->next;
    }

    while (l2 != NULL)
    {
        sum = l2->val + reminder;
        result = sum % 10;
        append(&output, result);
        reminder = sum / 10;

        l2 = l2->next;
    }

    if (reminder > 0)
    {
        append(&output, reminder);
    }

    return output;
    }