# you can use print for debugging purposes, e.g.
# print "this is a debug message\n";
use Data::Dumper;

sub solution {
    my ($L)=@_;
    print Dumper $L;
    # write your code in Perl 5.14
}
[download]

and got this output, which clarifies that the implementation is done with hashes of hashes

(sorry indentation lost while copying)

Running solution...

Compilation successful.

Example test:   [1, 1, 1, 1]
Output (stderr):
WARNING: producing output may seriously slow down your code!
Output:
[download]

$VAR1 = {
          next => {
                      next => {
                                  next => {
                                              next => undef,
                                              value => 1
                                            },
                                  value => 1
                                },
                      value => 1
                    },
          value => 1
        };

Reading the confusing text reveals that those lazy people didn't even try to use proper Perl terminology.

Pointer? Dictionary? Empty? WTF?

Thanks for your help LanX, once Known the data structure, everything make sense now!

use strict;
use warnings;

my $pt = { value => "D", next => undef }; 
my $pt2 = {value => "C", next => $pt};
$pt = {value => "B", next => $pt2};
my $L = { value => "A", next => $pt};
[download]

It took me less than 5 minutes to write the code to count the nodes, but I'll reveal it only later, once I am sure that this cannot used for other purpose.

My iterative solution subroutine prints out the node being visited and the node count as follows:

ABCD
4
[download]

A recursive solution would probably be even simpler, but I haven't tried so far.

Thanks Laurent, beginner lever is good for me. I actually worked in biology department, and learned Perl just for analyzing data generated from our Next generation sequencing project. I guess linked list will not be something I will use in my data analysis in near future since it seems quite complex to me..... Thanks a lot for your code,though!

Thanks for your explanation! So basically, it is just a data structure as hash of hashes or array of hash, right? Like in your example code, all scalar on the left are references. If I want to access the last element in the linked list, all I need to do is just keep deference it until the last one is reached, right? Something like $L->{'next'}->{'next'}->{'next'}->{'value'} right? ^_^