50 Pow(x, n)

https://leetcode.cn/leetbook/read/top-interview-questions-medium/xwo102/

cost: 11m26s

class Solution {
public:
    double myPow(double x, int n) {
        if (n < 0) {
            if (n == -0x7FFFFFFF - 1) {
                n = 0x7FFFFFFF;
                return 1 / (x * myPow(x, n));
            }
            n = -n;
            return 1 / myPow(x, n);
        }
        if (n == 0) {
            return 1;
        }
        if (n == 1) {
            return x;
        }
        if (n == 2) {
            return x * x;
        }
        if (n % 2 == 0) {
            double r = myPow(x, n / 2);
            return r * r;
        }
        double r = myPow(x, n / 2);
        return r * r * x;
    }
};