二分法求a^b（递归）

原理：b为偶数时，a^b = a^(b/2) * a^(b/2) = (a*a)^(b/2),如a^4 = a^2 * a^2  

b为奇数时，a^b = a * a^(b/2) * a^(b/2) =a *  (a*a)^(b/2),如a^5 = a * a^2 * a^2

    LL A_b(LL a, LL b)
    {
        if (b == 0)
        {
            return 1;
        }

        LL temp = A_b(a*a, b >> 1);
        if(b  & 1)
        {
            temp = temp*a;
        }
        return temp;    

    }