def rec(d, n, arr, res)
  if n <= 0
    Array.new(arr.length) {""}
  elsif d >= n
    res
  else
    rec(d + 1, n, arr, arr.product(res)
                           .select {|a, b| a != b[0]}
                           .map {|a, b| a + b})
  end
end

def generate(arr, n)
  rec(1, n, arr, arr)
end

print generate(['a', 'b', 'c'], 0)