+ // after first recursion, (*node_first)->prev->next == *node_first,
+ // so *node_first would be overwritten with (*node_first)->next
+ // => use a copy of *node_first (and later of (*node_first)->next)
+ ListNode *node = *node_first;
+ ListNode *node_next = node->next;
+
+ checked_free(node->key);