foo.cc: In function 'void pre()':
foo.cc:11:14: warning: iteration 1000008 invokes undefined behavior [-Waggressive-loop-optimizations]
   11 |   if (prime[i] == 0) b[++k] = i; //如果没有被标记为1，就是质数
      |       ~~~~~~~^
foo.cc:9:20: note: within this loop
    9 |  for (int i = 2; i <= N; i++)//找出N以内的素数并标记
      |                  ~~^~~~

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