foo.cc: In function 'void init()':
foo.cc:11:9: warning: iteration 10000008 invokes undefined behavior [-Waggressive-loop-optimizations]
   if(a[i]==0) prime[++cnt]=i;
      ~~~^
foo.cc:9:15: note: within this loop
  for(int i=2;i<=maxn;i++)
              ~^~~~~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。