palindrime.pl

palindrome.pl

#!/usr/bin/env perl
# perl note - if @ds = [1,2,3], shift(@ds) returns 1 and make @ds be [2,3]
#
# perl note - perl lets you treat chars and numbers the same and just mostly
# figures out what to do

use v5.38;

# the input
my $t = '8155999';

# the output should have the highest value - that means the
# highest number should be the first digit
#
# turn the string into a sorted array of digits
#
# reverse/sorting it will make sure the values decrease, and
# since it is sorted, we know we will see all of a certain
# number sequentially (i.e. below, we don't see a 9 appear at
# the end of the array)
my @ds = reverse(sort(split(//,$t))); # 8155999 -> [9,9,9,8,5,5,1]

# prepare the output variables -
# $left: the "left" side of the palindrome
# $middle: the "center" of the palindrome (could be empty!)
# $right: the "right" side of the palindrome
#
# why is $middle -1? to set up for replacing it with the biggest
# value found
my ($left,$middle,$right) = ('', -1, '');

# in pseudocode:
# while there are digits left in @ds {
#   if there are at least two digits and they are the same {
#     set $left to be $left . the first value
#     set $right to be the second value . $right
#   } else {
#     the next number does not have a "pair", so
#     it can either be made the $middle if it is
#     larger than $middle (which should only happen
#     once since the list is sorted)
#   }
# }
# print $left . $middle . $right
#

while ($#ds != 0) { # while @ds has elements
  if (($#ds != 1) && ($ds[0] == $ds[1])) { # matching pair
    ($left, $right) = ($left . shift(@ds), shift(@ds) . $right);
  } else { # only occurs once
    # because we reverse sorted the array, the first number
    # we see hear is the best one to pick, it is the largest
    $middle = $ds[0] if ($middle == -1);
    shift(@ds);
  }
}
say $left . $middle . $right;