/in/foo.cc: In function 'int main()':
/in/foo.cc:14:8: warning: iteration 1000000 invokes undefined behavior [-Waggressive-loop-optimizations]
   a[i] = ( a[i - 1] * i ) % 998244353;
   ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/in/foo.cc:13:19: note: within this loop
  for(int i = 1; i <= 1000001; i++)
                 ~~^~~~~~~~~~