krahets/hello-algo · error · IndexError

队列为空

Error message

队列为空

What it means

Raised by `LinkedListQueue#peek` (linkedlist_queue.rb:55) when `is_empty?` is true. The method reads `@front.val`; on an empty queue `@front` is nil, so the guard prevents a NoMethodError. The `pop` method delegates to `peek`, so dequeueing an empty queue also triggers this.

Source

Thrown at codes/ruby/chapter_stack_and_queue/linkedlist_queue.rb:55

      @rear.next = node
      @rear = node
    end

    @size += 1
  end

  ### 出队 ###
  def pop
    num = peek
    # 删除头节点
    @front = @front.next
    @size -= 1
    num
  end

  ### 访问队首元素 ###
  def peek
    raise IndexError, '队列为空' if is_empty?

    @front.val
  end

  ### 将链表为 Array 并返回 ###
  def to_array
    queue = []
    temp = @front
    while temp
      queue << temp.val
      temp = temp.next
    end
    queue
  end
end

### Driver Code ###
if __FILE__ == $0

View on GitHub (pinned to 69932aed18)

Solutions

1. Guard with `unless queue.is_empty?` before `peek` or `pop`.
2. Drive the consumer with `while queue.size > 0`.
3. Rescue IndexError if an empty peek is recoverable.

Example fix

# before
head = queue.peek

# after
head = queue.is_empty? ? nil : queue.peek

return nil if queue.is_empty?
queue.peek

def ll_queue_readable?(queue)
  queue.respond_to?(:is_empty?) && queue.respond_to?(:peek) && !queue.is_empty?
end

begin
  queue.peek
rescue IndexError
  nil
end

When it happens

Trigger: Calling `queue.peek` or `queue.pop` when the linked list has no nodes (`@size == 0`). Occurs after consuming all elements or before the first enqueue.

Common situations: BFS frontier fully drained then peeked; consumer loop that pops past empty; queue shared across threads where one drains before the other peeks.

Related errors

• очередь пуста
• 佇列為空
• очередь пуста
• 队列为空
• 双向队列为空

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/8499fe9ec31a96b6. Report an issue: GitHub.