不用 unsigned 也可以过
特殊处理边界，用减法
贴一小段核心代码
		int Start, Size = U - L + 1, i = 1;
		int p = primes[i], QRT = (int)sqrt(U) + 1;
		memset(LU,true,Size + 80);
		while (p <= QRT)
		{
			Start = ( p - (L - 1) % p ) - 1
			if(L < p*2 - Start);//这里不用加法判断，否则WA
				Start += p;

			if (Start % 2 == 1)
				Start += p;

			while (Start < Size)
			{
				if(LU[Start] == true)
					LU[Start] = false;
				Start += p*2;
			}
			p = primes[++i];
		}