foo.cc: In function ‘int main()’:
foo.cc:14:22: warning: iteration 1000000 invokes undefined behavior [-Waggressive-loop-optimizations]
   14 |                 a[i] = ( a[i - 1] * i ) % 998244353;
      |                 ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.cc:13:26: note: within this loop
   13 |         for(int i = 1; i <= 1000001; i++)
      |                        ~~^~~~~~~~~~