我有以下代碼：    List<String> list = Arrays.asList("a", "b", "c", "a", "c");    Map<String, List<String>> map = list.stream().collect(Collectors.groupingBy(    Function.identity() ,    Collectors.toCollection(ArrayList::new)    ));它產(chǎn)生的是  {a=[a, a], b=[b], c=[c, c]}很高興請(qǐng)問(wèn)我應(yīng)該如何編寫映射方法來(lái)獲取可以給我的Map：{a=2, b=1, c=2}