foo.cc: In function 'void pre()':
foo.cc:10:14: warning: iteration 999999 invokes undefined behavior [-Waggressive-loop-optimizations]
   10 |   if (prime[i] == 0) b[++k] = i; //如果没有被标记为1，就是质数。我接下来会讲解为什么是质数。
      |       ~~~~~~~^
foo.cc:8:20: note: within this loop
    8 |  for (int i = 2; i <= N; i++)//找出N以内的素数并标记
      |                  ~~^~~~

[Hydro](https://hydro.ac)提供评测服务