Pow(x, n)

Implement pow(x, n).

Analysis: 

public class Solution {
    public double power(double x, int n) {
        if(n==0) return 1;
        double y = power(x, n/2);
        if(n%2==0) return y*y;
        else return y*y*x;
    }
    
    public double pow(double x, int n) {
        if(n<0) return 1.0/power(x, -n);
        else return power(x, n);
    }
}