只需返回队列.您还必须测试输入的 queue 是否为NULL，在这种情况下，您显然无法访问 next 指针来查找结尾，因此在这种情况下，只需返回新节点.

Just return queue. You also have to test if the input queue is NULL, in which case you obviously can't access a next pointer to find the end, so in that case just return the new node.

Queue *enqueue(Queue *queue, int data){

    Queue *new_node, *p;

    new_node = malloc(sizeof(Queue));
    new_node->data = data;
    new_node->next = NULL;

    if (!queue)
        return new_node;

    p = queue;
    while (p->next)
        p = p->next;
    p->next = new_node;

    return queue;
}