27 数值的整数次方

class Solution {
public:
 double Power(double base, int exponent) {
    double minus = 1;//exponent正负标志位，负：-1
    if(exponent < 0) minus = -1,exponent=-exponent;
    double res = 1;
        while(exponent)
        {
            res = res * base;
            exponent--;
        }
        if(minus== -1) res = 1/res;//指数为负数，取倒数
        return res;
    }
};