50. Pow(x, n)

Implement pow(x, n).

 Notice

You don't need to care about the precision of your answer, it's acceptable if the expected answer and your answer 's difference is smaller than1e-3.

Have you met this question in a real interview? 

Yes

Example

Pow(2.1, 3) = 9.261
Pow(0, 1) = 0
Pow(1, 0) = 1

分析：

如果n是偶数，那么， power (x, n) = power(x,n/2) * power(x, n/2)。

如果n是奇数，那么， power (x, n) = power(x,n/2) * power(x, n/2) * x.

注意：当负数转成正数的时候，有可能会溢出。

public class Solution {
    public double myPow(double x, int n) {
        return helper(x, n);
    }
    
    private double helper(double x, long n) {
        if (n == 0 || x == 1) return 1;
        if (x == 0) return 0;
        
        if (n < 0) return 1 / helper(x, -n);
        
        double result = helper(x, n / 2);
        if (n % 2 == 0) {
            return result * result;
        }
        return result * result * x;
    }
}