/in/foo.cc: In function 'int main()':
/in/foo.cc:23:11: warning: iteration 999998 invokes undefined behavior [-Waggressive-loop-optimizations]
   if(!st[i]){
       ~~~~^
/in/foo.cc:22:15: note: within this loop
  for(int i=2;i<=N;i++){//求出所有小于1e6的质数 
              ~^~~