input = %w(a b c)
n = 5

def gen_seq(prefix, alphabet)
  alphabet.select { |y| y != prefix[-1] }.map { |y| "#{prefix}#{y}" }
end

def gen_seq_for_prefixes(prefixes, alphabet)
  prefixes.reduce([]) { |memo, l| memo.push(*gen_seq(l, alphabet)) }
end

(1...n)
    .reduce(input) { |memo| gen_seq_for_prefixes(memo, input) }
    .map { |x| puts "#{x}" }