50. Pow(x, n)

x^n      n = 7    =     111  = 2^2 + 2^1 + 2^0，则 x^n = x^(2^2 + 2^1 + 2^0) = x^(2^2)  * x^(2^1 )  * x^(2^0) 

public class Solution {
    public double myPow(double x, int n) {
        double res = 1.0;
        long n_l = Math.abs((long)n);
        while(n_l > 0){
            if((n_l & 1) == 1) res *= x;
            x *= x;
            n_l >>= 1;
        }
        
        return n >>> 31 == 1 ? 1 / res : res;
    }
}