50. Pow(x, n)

Implement pow(x, n).

题目含义：实现x的n次方

    private double power(double x,int n)
    {
        if (n == 0) return 1;
        double halfPower = power(x, n / 2);
        if (n % 2 == 0) return halfPower * halfPower;
        else return halfPower * halfPower * x;
    }
    
    public double myPow(double x, int n) {
        if (n<0) return 1/power(x,-n);
        return power(x,n);        
    }