java 输入数字求和 关键是计算出每一项的值

Question

题目:输入2个数字，数字一记做a,数字二记做b.求s = a +aa +aaa +aaaa+aa...a

Answer 1

public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        String a = scan.next();
        int b = scan.nextInt();
        String expr = cal(a,b,new StringBuilder(b), "");
        System.out.println(expr);
        System.out.println(MVEL.eval(expr));
    }

    public static String cal(String a,int b,StringBuilder expr,String sum){
        sum = sum + a;
        expr = expr.append(sum);
        if(sum.length()<b){
            expr.append("+");
            return cal(a,b,expr,sum);
        }
        return expr.toString();
    }