Implement pow(x, n).

Pow(x, n).cpp

class Solution {
public:
    double pow(double x, int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(n == 0) {
            return 1;
        }
        
        
        if(n < 0) {
            
            if(n == -2147483648) {
                return pow(1 / x, -(n + 1)) / x;
            }
            
            return pow(1 / x, -n);
        }
        
        double res = pow(x, n >> 1);
        if(n >> 1 << 1 == n) {
            x = res * res;
        } else {
            x = res * res * x;
        }

        return x;
    }
};