dist_square_sum.pl

sub dist_square_sum {
  my @array = @_;
  my ($sum, $sum_sq, $r);
  $sum = $sum_sq = $r = 0;
  for (@array) { # O(N)
    $sum    += $_; # O(1)
    $sum_sq += $_**2; # O(1)
  }
  for (0..$#array-1) { # O(N)
    $sum -= $array[$_]; # O(1)
    $r   += -2*$array[$_]*$sum; # O(1)
  }
  return (@array-1)*$sum_sq + $r; # O(1)

  # total: O(N)
}