/in/hasCircular.c:3:24: error: 'struct ListNode' declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
 int hasCircular(struct ListNode* head)
                        ^~~~~~~~
/in/hasCircular.c: In function 'hasCircular':
/in/hasCircular.c:5:10: error: dereferencing pointer to incomplete type 'struct ListNode'
  if (head->next == NULL)
          ^~
/in/hasCircular.c:13:1: error: control reaches end of non-void function [-Werror=return-type]
 }
 ^
cc1: all warnings being treated as errors