double quick_power(double x, unsigned n) {
    if (n == 0) return 1;
    if (n == 1) return x;
    return quick_power(x, n / 2)
        * quick_power(x, n / 2)
        * ((n & 1) ? x : 1);
}

T(N)=2*T(N/2)+O(1) ?