快速幂

LL PowerMod(LL a, LL b){
	if (b == 0) return 1;
	LL ans = 1;
	while (b != 0){
		if (b & 1) ans = (ans * a) % mod;
		b >>= 1;
		a = (a * a) % mod;
	}
	return ans;
}