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;
}