private void BTN_OverWrite_Click(object sender, EventArgs e)
        {
            if (!CheckInputBeingValid(TXT_NewEle.Text, TXT_NewVal.Text) ||
                !CheckIndexBeingValid(LST_Queue.SelectedIndex))
            {
                return;
            }

            QueueElement element = new QueueElement(TXT_NewEle.Text, TXT_NewVal.Text);

            if (elements.Count == 0)
            {
                elements.Add(element);
                LST_Queue.Items.Add(element);
            }
            else
            {
                if (!CheckIndexBeingValid(LST_Queue.SelectedIndex))
                {
                    return;
                }

                elements[LST_Queue.SelectedIndex]        = element;
                LST_Queue.Items[LST_Queue.SelectedIndex] = element;
            }
            UpdateFrontThreeElementDisplay();
        }
        private void BTN_Dequeue_Click(object sender, EventArgs e)
        {
            if (!CheckIndexBeingValid(LST_Queue.SelectedIndex))
            {
                return;
            }

            QueueElement temp = elements[LST_Queue.SelectedIndex];

            txtDeqEle.Text = temp.Element.ToString();
            txtDeqVal.Text = temp.Value.ToString();

            elements.RemoveAt(LST_Queue.SelectedIndex);
            LST_Queue.Items.RemoveAt(LST_Queue.SelectedIndex);
            UpdateFrontThreeElementDisplay();
        }