From ceff1eb36bffd4fa2602beba865b26f04a19b504 Mon Sep 17 00:00:00 2001
From: Tom Ryder <tom@sanctum.geek.nz>
Date: Sat, 17 Feb 2018 22:39:24 +1300
Subject: Initial commit

---
 binary-tree/binary-tree.pl      | 79 +++++++++++++++++++++++++++++++++++++++++
 linked-list/linked-list-demo.pl | 74 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 153 insertions(+)
 create mode 100644 binary-tree/binary-tree.pl
 create mode 100644 linked-list/linked-list-demo.pl

diff --git a/binary-tree/binary-tree.pl b/binary-tree/binary-tree.pl
new file mode 100644
index 0000000..9a23055
--- /dev/null
+++ b/binary-tree/binary-tree.pl
@@ -0,0 +1,79 @@
+#!/usr/bin/env perl
+
+use strict;
+use warnings;
+use utf8;
+
+use 5.006;
+
+# Start with an empty tree
+my $root = undef;
+
+# Read lines one by one
+while ( defined( my $line = <> ) ) {
+
+    # Create a new node with this line's value, and empty "left" and "right"
+    # references
+    my $new = {
+        line  => $line,
+        left  => undef,
+        right => undef,
+    };
+
+    # If the tree is empty, this can be our first node; skip the rest of this
+    # iteration
+    if ( not defined $root ) {
+        $root = $new;
+        next;
+    }
+
+    # Starting at the root ...
+    my $cur = $root;
+
+    # Until we don't have any more nodes for comparison (i.e. we've found the
+    # place for our new node)...
+    while ( defined $cur ) {
+
+        # Decide whether to add this to the "left" subtree or the "right"
+        # subtree, depending on whether it's less than the current node or not
+        my $dir =
+          $line lt $cur->{line}
+          ? 'left'
+          : 'right';
+
+        # If that reference is undefined, then that's where we'll put this new
+        # node; unset $cur so the loop stops
+        if ( not defined $cur->{$dir} ) {
+            $cur->{$dir} = $new;
+            $cur = undef;
+            next;
+        }
+
+        # Otherwise, we resume descent with the next node
+        $cur = $cur->{$dir};
+    }
+}
+
+# Start a stack of node items to fall back to once we've processed a node, and
+# begin at the root
+my @stack;
+my $cur = $root;
+
+# Keep going as long as there are nodes left in the stack or we have a
+# "current" node to print
+while ( @stack or defined $cur ) {
+
+    # Drill down through the "left" nodes, stacking them up, until we hit NULL
+    while ( defined $cur ) {
+        push @stack, $cur;
+        $cur = $cur->{left};
+    }
+
+    # If there is anything in the stack after doing that, pop off the first
+    # one, print it, and then move to its "right" node
+    if (@stack) {
+        $cur = pop @stack;
+        print "\t$cur->{line}";
+        $cur = $cur->{right};
+    }
+}
diff --git a/linked-list/linked-list-demo.pl b/linked-list/linked-list-demo.pl
new file mode 100644
index 0000000..069cb3c
--- /dev/null
+++ b/linked-list/linked-list-demo.pl
@@ -0,0 +1,74 @@
+#!/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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