krahets/hello-algo · error · Error

キューが空です

Error message

キューが空です

What it means

Thrown by pop() on the linked-list-backed queue when `this.front` is null. Defensive duplicate: pop() first calls peek() (which already throws 'キューが空です' when size===0), so reaching this line means the front pointer is null even though size is non-zero — an inconsistent internal state. Practically, callers see this only if peek's precondition somehow passed but front is still null.

Source

Thrown at ja/codes/typescript/chapter_stack_and_queue/linkedlist_queue.ts:49

    push(num: number): void {
        // 末尾ノードの後ろに num を追加
        const node = new ListNode(num);
        // キューが空なら、先頭・末尾ノードをともにそのノードに設定
        if (!this.front) {
            this.front = node;
            this.rear = node;
            // キューが空でなければ、そのノードを末尾ノードの後ろに追加
        } else {
            this.rear!.next = node;
            this.rear = node;
        }
        this.queSize++;
    }

    /* デキュー */
    pop(): number {
        const num = this.peek();
        if (!this.front) throw new Error('キューが空です');
        // 先頭ノードを削除
        this.front = this.front.next;
        this.queSize--;
        return num;
    }

    /* キュー先頭の要素にアクセス */
    peek(): number {
        if (this.size === 0) throw new Error('キューが空です');
        return this.front!.val;
    }

    /* 連結リストを Array に変換して返す */
    toArray(): number[] {
        let node = this.front;
        const res = new Array<number>(this.size);
        for (let i = 0; i < res.length; i++) {
            res[i] = node!.val;

View on GitHub (pinned to 69932aed18)

Solutions

1. Guard pop() with `if (queue.size > 0)` (or isEmpty if exposed) — this prevents the peek throw, which is the realistic path.
2. Avoid mutating front/rear/queSize from outside the class; treat the list as opaque.
3. If subclassing, preserve the front===null ⇔ queSize===0 invariant.

Example fix

// before
const v = queue.pop();

// after
const v = queue.size > 0 ? queue.pop() : undefined;

// Guard queue pop (peek throws first with the same message on empty).
if (queue.size > 0) {
  const v = queue.pop();
}

function linkedQueueCanPop(queue) {
  return queue.size > 0;
}

try {
  const v = queue.pop();
} catch (e) {
  if (e instanceof Error && e.message === 'キューが空です') {
    // empty (or invariant violated)
  } else throw e;
}

When it happens

Trigger: Calling pop() on an empty queue (peek throws first with the same message); corrupting internal state by manipulating front/queSize directly; subclassing LinkedListQueue and bypassing encapsulation.

Common situations: Normal empty-queue pop surfaces the same message from peek(); the pop-level check is a safety net for invariant violation. Debugging this exact line usually points at external mutation of front/rear/queSize fields.

Related errors

• 队列为空
• スタックが空です
• очередь пуста
• 队列为空
• 队列为空

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/9bbe6509e0977708. Report an issue: GitHub.