binary strategy: 2^n=2^(n/2) * 2^(n/2) * 2^(n%2)

Pow(x, n).java

public class Solution {
    public double pow(double x, int n) {
        // Start typing your Java solution below
        // DO NOT write main() function        
      if(n==0) return 1; 
      if(x==0) return 0;
      if(n==1) return x; 
      if(n<0) 
      {
	n=-n; x=1/x; 
	}
	double half=pow(x, n/2); 
	double result=half*half*pow(x, n%2);
	return result; 
    }
}