Implement pow(x, n).

some consideration:

• what if n < 0
• avoid recompute

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