输入字符串“abc“，输出组合a,b,c,ab,ac,bc,abc

输入字符串"abc"，输出组合a,b,c,ab,ac,bc,abc

class Main {
    public static void main(String[] args){
        String s = new Scanner(System.in).next();
        combine(s);
    }

    public static void combine(String str){
        char[] in=str.toCharArray();
        StringBuilder out = new StringBuilder();
        List<String> list = new ArrayList<>();//将组合添加至list中，排序。按照a,b,c,ab,ac,abc输出
        allCombine(in,out,0,list);
        list.stream().forEach(s -> System.out.print(s+" "));
    }
    
    public static void allCombine(char[] ch, StringBuilder sb, int start, List<String> list){
        int len=ch.length-1;
        for(int i=start;i<=len;i++){
            sb.append(ch[i]);
            //System.out.print(sb+" "); =>不添加至list中，直接打印 a,ab,abc,abcd,ac,acd,b,bc,bd,d
            list.add(sb+"");
            Collections.sort(list,(o1, o2) -> o1.length() - o2.length());
            if(i<len){
                allCombine(ch,sb,i+1,list);
            }
            sb.setLength(sb.length()-1);//执行完abcd后，回溯 -> abc -> ab重新执行 abd
        }
    }
}