foo.cc: In function 'int main()':
foo.cc:24:11: warning: iteration 999998 invokes undefined behavior [-Waggressive-loop-optimizations]
   if(!st[i]){
       ~~~~^
foo.cc:23:15: note: within this loop
  for(int i=2;i<=N;i++){//求出所有小于1e6的质数 
              ~^~~

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