template <class T>
void HeapSort(T a[], int n)
{// Sort a[1:n] using the heap sort method.
   MaxHeap<T> H(1);
   H.Initialize(a,n,n);
   T x;
   for (int i = n-1; i >= 1; i--) { H.DeleteMax(x); a[i+1] = x; }
   H.Deactivate();
}