Permuations of a string

def permutations(str, prefix)
  if str.length == 0
    puts prefix
  else
    (0).upto(str.length-1) do |i|
      permutations(str.slice(0,i)+str.slice(i+1, str.length), prefix + str[i])
    end
  end
end

puts permutations("abcd", "")