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This is a medium difficulty Coding & Algorithms question, commonly asked during Technical Screen rounds at Coinbase.

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This question is commonly asked for Software Engineer candidates at Coinbase during technical interviews.

Paginate forward and backward through results

Company: Coinbase

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: medium

Interview Round: Technical Screen

You are asked to extend a basic pagination system to support **forward** and **backward** navigation, similar to a cursor-based API. You are given: - A zero-indexed array of items `items` sorted in a fixed, deterministic order. - An integer `pageSize > 0`. Design a paginator with the following operations: 1. `getFirstPage()` → returns the first `pageSize` items and a **cursor** representing the position after the last returned item. 2. `getNextPage(cursor)` → given a cursor returned by a previous call, return the next `pageSize` items and a new cursor. If there are no more items, return an empty list and a special "end" cursor. 3. `getPrevPage(cursor)` → given a cursor, return the **previous** page of `pageSize` items and a new cursor pointing to the start of that page. If there is no previous page, return an empty list and a "start" cursor. Requirements: - Define what the cursor contains (it could just be an index or a more complex token). - All operations should run in **O(pageSize)** time. - Handle edge cases correctly, including: - Moving past the last page with `getNextPage`. - Moving before the first page with `getPrevPage`. - Cases where `len(items)` is not a multiple of `pageSize`. Implement the data structure and its methods with clear handling of indices and cursors.

Quick Answer: This question evaluates understanding of pagination mechanics, cursor/token design, array indexing, and state management for bidirectional (forward and backward) navigation.

Implement solution(items, page_size, operations). A cursor is the integer index after the last item returned by a page. Operations are ["first"], ["next", cursor], and ["prev", cursor]. Return [page_items, new_cursor] for each operation.

Constraints

page_size > 0
items are already sorted in deterministic order

Examples

Input: (['a', 'b', 'c', 'd', 'e'], 2, [['first'], ['next', 2], ['next', 4], ['next', 5], ['prev', 5], ['prev', 1], ['prev', 0]])

Expected Output: [[['a', 'b'], 2], [['c', 'd'], 4], [['e'], 5], [[], 5], [['d', 'e'], 3], [['a'], 0], [[], 0]]

Input: ([1, 2, 3], 5, [['first'], ['next', 3], ['prev', 3]])

Expected Output: [[[1, 2, 3], 3], [[], 3], [[1, 2, 3], 0]]

Input: ([], 3, [['first'], ['next', 0], ['prev', 0]])

Expected Output: [[[], 0], [[], 0], [[], 0]]

Hints

Use the cursor as an array boundary.
Clamp invalid cursors into [0, len(items)].

Quick Overview

This question evaluates understanding of pagination mechanics, cursor/token design, array indexing, and state management for bidirectional (forward and backward) navigation.