ruby array hash get key by value and value by key

I have hash{ "1" => "one", "2" => "two", "3" => "three" }

How to get key by value and value by key.

if I pass "1" I get "one", if I pass "two" I get "2"

def finder (string)
  table = { "1" => "one", "2"   =>  "two", "3"  =>  "three" }
  *magick*
  }
  return string
end
puts finder("1") => one
puts finder("one") => 1