#!/usr/bin/env perl -l

use strict;
use warnings;
use List::Compare;

my %tree_path_for;
my %is_known_node;
my @nodes;

while (<DATA>) {
    my ($node, $parent) = split;

    ++$is_known_node{$node};
    $tree_path_for{$node}
        = [$node => $parent eq 'root' ? () : \$tree_path_for{$parent}];
}

for (@ARGV) {
    if ($is_known_node{$_}) {
        push @nodes => $_;
    }
    else {
        print "Unknown node: $_";
    }
}

if (! @nodes) {
    print "No valid nodes to process!";
}
else {
    print "Nodes for this run: @nodes";

    if (@nodes == 1) {
        print "Need at least two nodes to process!";
    }
    else {
        my @commons = List::Compare::->new(
            map { [ flatten_nested_AoA($tree_path_for{$_}) ] } @nodes
        )->get_intersection;

        if (@commons) {
            print "Lowest common ancestor: ", $commons[-1];
        }
        else {
            print "No common ancestor!";
        }
    }
}

sub flatten_nested_AoA {
    map { ref $_ ? flatten_nested_AoA($$_) : $_ } @{+shift}
}

# Example ancestor trees
#
#     _A_               W
#    /   \              |
#   B   __C__           X
#      /  |  \         / \
#     D   E   F       Y   Z
#        / \
#       G   H

# Data: node parent
#
__DATA__
G E
D C
C A
A root
E C
H E
Y X
F C
X W
Z X
W root
B A