double ** dmatalloc(size_t nrows, size_t ncols) {
  double ** mat = malloc( nrows*sizeof(double*) + nrows*ncols*sizeof(double) );
  mat[0] = (double *) &mat[nrows];
  for(size_t ii = 1; ii < nrows; ++ii) {
        mat[ii] = &mat[ii-1][ncols];
  }
  return mat;
}

void dmatfree(double** mat) {
        free(mat);
}

static void reverse_helper(char * str, size_t ii, size_t jj) {
  // Exchange characters
  char tmp = str[ii];
  str[ii]  = str[jj];
  str[jj]  = tmp;
  // Check next move
  ii++;
  jj--;
  if( ii >= jj) {
    return;
  }
  reverse_helper(str,ii,jj);
}

// Requires a '\0' terminated string
void reverse(char * str) {
    size_t str_length = 0;
    while( str[str_length] != '\0') {
        str_length++;
    }
    if (str_length == 0) return;
    reverse_helper(str,0,str_length-1);
}