假如有一个String的数组 如a为{"a","a","b","a","c","b","a","c","c","d"},现在我想把a数组中的相同字符抽取出来,放到一个信息的ArrayList集合中b中,即如果b有4个相同的字符,则b中的元素为4个string的数组。请帮忙写一个方法去实现,谢谢