ListNode * head = new ListNode(-1);
push(head, 5);
push(head, 20);
push(head, 4);
push(head, 3);
push(head, 30);
printf("%x", head);
printList(head);
printf("%x", head);

void printList(ListNode  *head) {
while (head != NULL) {
    printf("%d ", head->val);
    head = head->next;
}
printf("n");
}