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#!/usr/bin/env perl
use strict;
use warnings;
use utf8;
use 5.006;
# Two pointers: one to the head of the list, one to the "current" node
my $head = undef;
my $cur = undef;
### Build the list:
# Read lines of text, and add a new node to the list each time
while ( defined( my $line = <> ) ) {
# Make a new node: a hashref with two fields; one with our read line, and
# one pointing to the "next" node, and there isn't one yet, so we'll make
# that undef
my $new = {
line => $line,
next => undef,
};
# If $head doesn't yet point anywhere, this is the first node in our list,
# so Make that our starting point
if ( not defined $head ) {
$head = $new;
}
# Otherwise, we make the new node the "next" one of the last one we created
else {
$cur->{next} = $new;
}
# The node we just created then becomes our "current" node.
$cur = $new;
}
### Print the list:
# Starting at $head, print the node's text and follow its "next" until it's
# "undef", i.e. we've reached the end of the chain.
print "Pre-insert:\n";
for ( $cur = $head ; defined $cur ; $cur = $cur->{next} ) {
print "\t$cur->{line}";
}
### Insert into the list:
# Add a node with string "lmao" after any node with string "ayy"
for ( $cur = $head ; defined $cur ; $cur = $cur->{next} ) {
# Just keep walking if we don't match
next if $cur->{line} ne "ayy\n";
# Make a new node, and make its next node whatever the current node's
# "next" was going to be; we're stealing its link, inserting it into the
# chain.
my $new = {
line => "lmao\n",
next => $cur->{next},
};
# Re-point the current node's "next" to the node we just created.
$cur->{next} = $new;
}
### Print the list again:
print "Post-insert:\n";
for ( $cur = $head ; defined $cur ; $cur = $cur->{next} ) {
print "\t$cur->{line}";
}
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