Implement pow(x, n). http://leetcode.com/onlinejudge#question_50

leetcode_pow.cpp

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