算法设计——计算a的n次方

double pow (double a, int n){
    if (n == 0) return 1;
    if (n == 1) return n;
    double t = pow(a, n/2);
    return t * t * pow(a, n%2);
}

double pow (double a, int n){
    double ret = 1;
    while (n) {
        if (n%2 == 1) ret *= a;
        a *= a; n /= 2;
    }
    return ret;
}