剑指 Offer 16. 数值的整数次方

快速幂。

class Solution {
public:
    double qpow(double x, long long n) {
        double res = 1;
        while (n) {
            if (n & 1) res *= x;
            x = x * x;
            n >>= 1;
        }
        return res;
    }

    double myPow(double x, int n) {
        return n >= 0 ? qpow(x, n) : 1.0 / qpow(x, -(long long)n);
    }
};