(list)+general style change, use unary switches for checking null

list.cpp

@@ -45,7 +45,7 @@ List<T> *append(List<T> *lst, T value)
 template <typename T>
 std::ostream& operator<<(std::ostream& ostream, const List<T> *lst)
 {
-  if (lst == NULL)
+  if (!lst)
     return ostream;
   ostream << "|" << lst->value << lst->next;
   return ostream;