Isomorphic strings

def isomorphic(a,b)
  if a.length == 0 && b.length == 0
    true
  elsif a.length != b.length
    false
  else

    a_stru = b_stru = Hash.new{|hash, key| hash[key] =[] }
    b_stru = Hash.new{|hash, key| hash[key] =[] }

    a_chars = a.split("")
    b_chars = b.split("")

    a_chars.each_with_index do |value, i|
      a_stru[value] << i
      b_stru[b_chars[i]] << i
    end

    a_stru = a_stru.values
    b_stru = b_stru.values

    a_stru.each_with_index.all?{|value, key|  b_stru[key] == value }

  end
end