In constructor 'LinkList::LinkList()',
    inlined from 'int main()' at /in/foo.cc:48:11:
/in/foo.cc:20:29: warning: 'l.LinkList::tail' is used uninitialized [-Wuninitialized]
   20 |                 head ->next=tail;
      |                             ^~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:48:18: note: 'l' declared here
   48 |         LinkList l;
      |                  ^