Question

I have a list of strings and I need to remove all items that are unique or has more than one duplicate. Other words, I need just those items, that are in the list exactly twice. How can I do it in Perl?

Answer 1

It’s a special case of the classic remove duplicates recipe. When your items are strings, this is really easy in Perl 5:

my %seen;
for my $item (@list) {
    $seen{$item}++;
}

my @twice = grep { $seen{$_} == 2 } keys(%seen);

It’s O(N) on the size of the list, but I don’t think you can get away with doing this kind of thing without looking at every element at least once…