my @array = ( 1, 2, 3, 4, 5, 6 );
my $func = sub { $_[0] <=> $_[1] };

my %tree = buildtree( $func, @array );  # the golf function

# one possible output :
%tree => { 2 => [
                  { 1 => [ 0, 0 ] } ,
                  { 4 => [
                           { 3 => [ 0, 0 ] },
                           { 5 => [ 0, 
                                    { 6 => [ 0, 0 ] }
                                  ]
                           }
                         ]
                  }
                ]
            }