Print out all unique permutations of first N natural numbers

permutations.rb

def permutations(n)
  _p n-1, (1..n).to_a, ''
end

def _p(n, arr, res)
  if n == -1
    puts res
    return
  end
  (0..n).each do |m|
    res2 = '' + res
    arr2 = arr.clone
    val = arr2.slice!(m)
    res2 += ".#{val}"
    _p(n - 1, arr2, res2)
  end
end