--- invert

    ͤ饭ؤΥϥå֤ޤ
    ۤʤ륭ФͤϿƤη̤Ǥ뤳Ȥ
    դƤΤ褦ʾˤΥ᥽åɤѤ뤳Ȥϰտޤ
    Ƥޤ

        h = { "n" => 100, "m" => 100, "y" => 300, "d" => 200, "a" => 0 }
        h.invert   #=> {200=>"d", 300=>"y", 0=>"a", 100=>"n"}

