private void Sort(ObservableArray tab, int left, int right) {
        if (left < right) {
            int center = (left + right) / 2;
            // sort left
            Sort(tab, left, center);
            // sort right
            Sort(tab, center + 1, right);
            // Merge the two array
            Merge(tab, left, center, right);
        }

    }