农场里有一群奶牛,每头奶牛都有一个编号。编号由小写字母组成,长度为n。为了方便管理,农场主想把奶牛们按照编号进行组合。请你求出所有可能的组合方式,要求组合中不得出现重复的编号,返回按照字典序排列。
示例1

输入

"abc"

输出

["","a","ab","abc","ac","b","bc","c"]
示例2

输入

"aabb"

输出

["","a","aa","aab","aabb","ab","abb","b","bb"]

备注:
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