private static void merge(Comparable<?> a[], Comparable<?> aux[], int lo, int mid, int hi) {
assert Helper.isSorted(a, lo, mid);
assert Helper.isSorted(a, mid + 1, hi);
for (int k = lo; k <= hi; k++) aux[k] = a[k];
int i = lo, j = mid + 1;
for (int k = lo; k <= hi; k++) {
if (i > mid) a[k] = aux[j++];
else if (j > hi) a[k] = aux[i++];
else if (Helper.less(aux[j], aux[i])) a[k] = aux[j++];
else a[k] = aux[i++];
}
}
