subset子集

public class Subset {
    public static void main(String[] args) {
        set("","abc");
    }
    
    public static void set(String left, String right) {
        if(right == null || right.equals("")) {
            System.out.println(left);
            return;
        }
        
        //选
        set(left+right.substring(0,1),right.substring(1));
        //不选
        set(left,right.substring(1));
        
    }

}