I have longer hair than her.I have longer hair than she.I have longer hair than hers.哪个正确啊?

1,2 两句对(不过常用第1句）
第三句错的原因是不能把人与头发相比.可以这样说 My hair is longer than hers.Ｉhave longer hair than she does.I have longer hair than she has.My hair is longer than hers.再请问哈这几个句子有没有不对的？错在哪？My hair is longer than her.(than后接宾格，这个对吗？）第一、三肯定定对，第二句（Ihave longer hair than she has）没有语法错，但一般不用，常用第一句代替.第四句一定错，因为相比的两者不同类，my hair 与 her 不同类，如果 把HER改为hers就对