foo.cc: In constructor ‘LinkList::LinkList(int)’:
foo.cc:20:43: warning: passing NULL to non-pointer argument 1 of ‘Node::Node(int)’ [-Wconversion-null]
   20 |                 Node* tail = new Node(NULL);
      |                                           ^
foo.cc:7:18: note:   declared here
    7 |         Node(int x):data(x),next(NULL){}
      |              ~~~~^
foo.cc:20:23: warning: unused variable ‘tail’ [-Wunused-variable]
   20 |                 Node* tail = new Node(NULL);
      |                       ^~~~