Implement pow(

x, 

n).

[解题思路]

二分法，注意n<0的情况。

[Code]

1:    double power(double x, int n)  2:    {  3:       if (n == 0)  4:         return 1;  5:       double v = power(x, n / 2);  6:       if (n % 2 == 0)  7:         return v * v;  8:       else  9:         return v * v * x;  10:     }  11:     double pow(double x, int n) {  12:       // Start typing your C/C++ solution below  13:       // DO NOT write int main() function  14:       if (n < 0)  15:         return 1.0 / power(x, -n);  16:       else  17:         return power(x, n);      18:     }