求解一个字符串的所有字符的组合 Java代码

package day_10_1;
import java.util.*;

public class Main {

    public static  Set<String> stringSet = new TreeSet<>(new CompareAccordToStringLength());

    public static void main(String[] args) {
        getAllGroup("abc");
        System.out.println(stringSet.toString());
    }

    public static Set<String> getAllGroup(String str){
           return getAllGroupUnit(str, stringSet, 0);
    }

    public static Set<String> getAllGroupUnit(String str,  Set<String> stringSet, int begin){
        if (str.length() - 1 == begin){
            stringSet.add(str.charAt(begin) + "");
        }else{
            Set<String> set = getAllGroupUnit(str, stringSet, begin + 1);
            combineStr(set, str.charAt(begin));
            stringSet.add(str.charAt(begin) + "");
        }
        return stringSet;
    }

    private static void combineStr(Set<String> stringSet, char a){
        if (null != stringSet && !stringSet.isEmpty()){
          ArrayList<String> strings = new ArrayList<>(stringSet);
            for (String str : strings){
                stringSet.add(a + str);
            }
        }

    }



}

class CompareAccordToStringLength implements Comparator<String>{

    @Override
    public int compare(String o1,String o2){

        if (o1.length() > o2.length()){
            return 1;
        }else{
           if (o1.length() == o2.length()){
               return o1.compareTo(o2);
           }
            return -1;
        }
    }
}