krahets/hello-algo · error · Error

Stack is empty

Error message

Stack is empty

What it means

Thrown by LinkedListStack.pop when stackPeek is null after peek. As with the queue case, this is a redundant defensive guard: pop calls peek() first, which already throws 'Stack is empty' when stackPeek is null. This branch is only reachable if stkSize and stackPeek are inconsistent. Treat it as the same empty-stack precondition as error 75.

Source

Thrown at en/codes/typescript/chapter_stack_and_queue/linkedlist_stack.ts:39

    }

    /* Check if the stack is empty */
    isEmpty(): boolean {
        return this.size === 0;
    }

    /* Push */
    push(num: number): void {
        const node = new ListNode(num);
        node.next = this.stackPeek;
        this.stackPeek = node;
        this.stkSize++;
    }

    /* Pop */
    pop(): number {
        const num = this.peek();
        if (!this.stackPeek) throw new Error('Stack is empty');
        this.stackPeek = this.stackPeek.next;
        this.stkSize--;
        return num;
    }

    /* Return list for printing */
    peek(): number {
        if (!this.stackPeek) throw new Error('Stack is empty');
        return this.stackPeek.val;
    }

    /* Convert linked list to Array and return */
    toArray(): number[] {
        let node = this.stackPeek;
        const res = new Array<number>(this.size);
        for (let i = res.length - 1; i >= 0; i--) {
            res[i] = node!.val;
            node = node!.next;

View on GitHub (pinned to 69932aed18)

Solutions

1. Check size() === 0 (or isEmpty if exposed) before pop.
2. Drain with while (stack.size() > 0).
3. Only mutate the stack through its public API.

Example fix

// before
const v = stack.pop(); // throws if empty

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

const v = stack.size() === 0 ? undefined : stack.pop();

function hasElements(s) { return typeof s.size === 'function' && s.size() > 0; }

try { return stack.pop(); }
catch (e) { if (!/Stack is empty/.test(e.message)) throw e; return undefined; }

When it happens

Trigger: Popping an empty stack (normally surfaced via peek first); corrupted state where stkSize > 0 but stackPeek is null.

Common situations: Calling pop without checking the stack depth; manual mutation of internal pointers; unbalanced push/pop in algorithm code.

Related errors

• Stack is empty
• Queue is empty
• 栈为空
• 栈为空
• Heap is empty.

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/7176357ec0efc2d8. Report an issue: GitHub.