void quicksort( int N, double v[]) {
  int i, j;
  double separator;
  if (N<2) return;
  separator = v[N/2];  
  i=0;
  j=N-1;
  while (j>i) {
    while ( v[i]<separator ) i++;
    while ( v[j]>separator ) j--;
    if (i<j) {
      scambia( v[i], v[j] );
      i++;
    }
  } 
  quicksort(i,v);
  quicksort(N-i,v+i);
}