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);    }}