分治法-pow(x,n)

    public static double myPow(double x,int n)
        {
            if (n < 0) return 1.0 / power(x, -n);
            else return power(x, n);

        }
        static double power (double x,int n)
        {
            if (n == 0) return 1;
            double v = power(x, n / 2);
            if (n % 2 == 0) return v * v;
            else return v * v * x;
        }