快速幂

ll quick_pow( ll x, ll y ) {
	if(x==0&&y==0) return 0;
	ll sum = 1 ;
	while( y > 0 ) {
		if( y % 2 )
			sum = sum * x % MOD ;
		y /= 2;
		x = x * x % MOD ;
	}
	return sum;
}