n次方根牛迭

    //n次方根牛迭
    public static double nSqrt(int a, int n) {
        double v = a;
        double last = 0;
        do {
            last = v;
//            v = (Math.pow(v, n) * (n - 1) + a) / (n * Math.pow(v, n - 1));
            v = ((n - 1) * v + (a / Math.pow(v, n - 1))) / n;
        } while (Math.abs(v - last) > 0.000001);
        return v;
    }