/in/foo.cc: In function 'int main()':
/in/foo.cc:13:19: warning: converting to non-pointer type 'char' from NULL [-Wconversion-null]
  char a[1999]{NULL};
                   ^
/in/foo.cc:19:23: warning: NULL used in arithmetic [-Wpointer-arith]
   if (a[2 * i + 1] == NULL)
                       ^~~~