void mergeSort(Item a[], int lo, int hi)
{
   int i;
   Item *aux = malloc((hi+1)*sizeof(Item));
   for (i = lo; i <= hi; i++) aux[i] = a[i];
   doMergeSort(a, aux, lo, hi);
   free(aux);
}
void doMergeSort(Item a[], Item b[], int lo, int hi)
{
   if (lo >= hi) return;
   int mid = (lo+hi)/2;
   doMergeSort(b, a, lo, mid);
   doMergeSort(b, a, mid+1, hi);
   merge(b+lo, mid-lo+1, b+mid+1, hi-mid, a+lo);
}