Problem Library

You have just been handed the wordlab repo. Read it before you change it.

You have just been handed the ledger repo, the audit tools the finance team runs at month end. Read it before…

You have just been handed the ridgeline repo, which produces the numbers in a printed trail guide. Read it…

The logstream ingest pipeline has two open items against it, and you did not write any of it.

The uptime service produces the "longest clean run" figure on a public status page. There are two open items…

The badge service reads the door log from the lobby turnstiles and produces the "repeat visitors" figure on a…

Support has two complaints open against the checkout service. Both are about money, and both are reproducible…

Payroll has a ticket open against the timecard service: three people were underpaid last week and everyone…

The lockfile service runs a nightly job that proposes package upgrades. It has a ticket open against it…

A teammate has added pagination and sorting to the orders endpoint and asked you to review it.

A teammate is cleaning up the users table and has asked you to review the migration before it runs on…

A teammate has opened a pull request against the auth service and asked you to review it.

A teammate assigned a ticket to a coding agent: "Implement the users CSV export for admins. The route is…

A teammate has added retries to the webhook delivery worker and asked you to review it.

The payments team has rebuilt checkout and wants to send a small share of customers to it first. A teammate…

A teammate has put a cache in front of the permission check, which every request goes through, and asked you…

The weekly digest email used to be sent by a script someone ran by hand on Mondays. A teammate has moved it…

Given a string s, return True if it is a palindrome after removing all non-alphanumeric characters and…

Given a list of numbers nums and a window size k, return a list of moving averages. Each element i in the…

Given a string of text, return the top n most frequent words as a list of [word, count] pairs, sorted by…

Format a number with thousands separators (commas) and a specified number of decimal places. Handle negative…

Given a list of raw phone-number strings pulled from a messy CRM export, return a new list containing each…

Turn a list of article titles into clean, web-safe URL slugs.

Parse a single line of CSV text into a list of field values.

Write a function that takes a nested configuration object and returns a flat object whose keys are the full…

You are given a list of order records. Each record is an object with two fields:

Your log ingestion pipeline receives event timestamps as strings. Before they hit the time-series store, you…

Write a redactor that masks sensitive values before records hit your logs.

You are given a list of contact records. Each record is a dictionary/object with three keys:

Your support inbox is a pile of free-text ticket bodies, and the team wants a quick read on what customers…

Payment systems store money as integer minor units to avoid floating-point error. A USD amount of $12.34 is…

A rate limiter divides time into back-to-back fixed windows of window_size seconds, anchored at epoch zero…

A feature-flag service resolves a flag's effective value by stacking configuration layers from least specific…

Given a list of integers nums, return a list of all elements that appear more than once.

You are given a CSV string representing sales data with columns: product, quantity, price.

Write a function flatten(obj) that converts a deeply nested dictionary (with nested dicts and lists) into a…

Write a function chunk_text(text, max_words, overlap=0) that splits a long text into chunks suitable for…

Given a string containing bracket characters ()[]{}<>, determine if the brackets are properly nested and…

Convert a subset of Markdown to HTML. Support: headings (#, ##, ###), bold (text), italic (text), inline code…

Given a list of daily stock prices and an integer k (maximum number of buy-sell transactions), return the…

Implement an in-memory URL shortener with two functions: - encode(long_url) → returns a short alphanumeric…

Given a dictionary mapping package names to their list of dependencies, return a valid installation order…

Implement two functions:

Implement two functions:

Given a JSON config object and a schema dictionary, validate the config and return a list of error strings…

Implement a token bucket rate limiter class. The bucket refills at rate tokens per second up to capacity.

Implement a StreamStats class that processes a stream of numbers and maintains running statistics without…

Write a function that reconciles a list of internal ledger entries against a bank feed. Two entries are…

Given a list of available semantic-version strings and a single version range, return the highest available…

Compute the sales tax owed on an invoice, line by line and in total, using a set of category-based tax rules.

You are building an anomaly detector for a time series of per-minute error counts. For each minute, you look…

Given two JSON values old and new, compute a structural diff: a list of every leaf-level difference between…

You are building the auto-scheduler behind a "find a time" button. Every attendee hands you a list of the…

An HR data export gives you a flat list of employees. Each record has a unique id and a manager_id pointing…

Build the delay schedule for a capped exponential backoff retry policy with deterministic jitter.

Mailing addresses arrive from many sources written in inconsistent styles: "123 Main St", "123 Main Street"…

Build a cursor-based pagination function over an already-sorted, in-memory dataset.

A build system runs a set of named tasks. Some tasks must finish before others can start, expressed as…

A checkout engine receives a shopping cart and a list of discount rules, and must compute the final order…

Implement a function that, given a standard five-field cron expression and a reference Unix timestamp…

Product analytics tools roll raw clickstream events up into sessions so that downstream charts can answer…

You are building the validation layer for an API gateway. Incoming JSON payloads must be checked against a…

A logistics rating engine routes outbound packages into shipping zones based on the leading digits of each…

You are given two metric series, series_a and series_b. Each series is a list of [timestamp, value] pairs…

A token bucket rate limiter holds at most capacity tokens. Tokens refill continuously at refill_rate tokens…

Social feeds need to turn raw post text into structured entities so the UI can render clickable mentions…

Build a weekly cohort retention matrix from raw signup and activity logs.

Feature-flag platforms ship features gradually by exposing them to a growing slice of users. The decision of…

When a client requests a partial download, it sends a Range request header. Your job is to parse that header…

A support ticket opens at a given timestamp and carries a service-level agreement (SLA) measured in business…

Company records arrive from dozens of messy sources, so the same business shows up many times under slightly…

A customer sends a single lump-sum payment, and your billing system must decide which of their open invoices…

Storefronts render a navigational breadcrumb so shoppers can see where a page sits inside the category tree…

Run-length encoding (RLE) is a simple lossless compression scheme that replaces runs of the same repeated…

Bundlers like Vite and webpack let projects declare path aliases so that an import like @/utils maps to a…

You are given a stream of card transactions, each a dictionary with the keys id, merchant, amount (an integer…

A GitHub-flavored Markdown (GFM) table is a delimited text block with three parts:

A reverse proxy sits in front of a pool of backend servers. Each backend is assigned an integer weight that…

A scheduling backend stores reservations against shared resources (rooms, desks, tables). Each resource maps…

A search backend needs every product name reduced to a canonical form so that a shopper typing cafe creme…

Design a data structure that follows the constraints of a Least Recently Used (LRU) cache.

You are given a multi-line string of web server logs in Apache Combined Log Format:

Implement a sliding window rate limiter that tracks API requests per user.

Implement a Trie (prefix tree) data structure with the following methods:

You are given a list of user events. Each event is a dictionary with: - user_id (string) - event_type…

Write a function evaluate(expression) that parses and evaluates a mathematical expression string.

Given a weighted directed graph and a list of source nodes, find the shortest distance from any source to…

Implement a Bloom filter with configurable false-positive rate.

Implement an event sourcing engine with snapshotting support.

Build a set of composable parser primitives, then use them to parse simple arithmetic expressions.

Implement an LRU cache extended with per-key TTL and cache statistics.

Build a composable, lazy data pipeline. A Pipeline chains stages that process items from an input iterable.

Implement the circuit breaker design pattern with three states: - CLOSED — normal operation, calls pass…

Build a semantic cache for LLM responses using TF-IDF cosine similarity — no external ML libraries allowed.

Parse and execute simple SQL queries against in-memory data (list of dicts).

Implement a consistent hashing ring with virtual nodes for load distribution.

Fold an event log into the current state of every entity.

Build a checkout helper that selects the most valuable combination of promo codes a customer can apply to a…

Given streams, a list of K already time-sorted log streams, merge them into one globally time-ordered stream.

You are given a chronological list of transactions for a single security. Each transaction is an object with…

A picker walks a warehouse laid out as a grid of parallel aisles (rows) divided into columns (slots along…

You maintain a key/value configuration object. Two collaborators each branched off the same starting point…

You maintain the discount engine that sits between the cart and checkout. Merchants define an ordered list of…

A card's authorization stream is just a list of transactions, each with a Unix timestamp (ts, in seconds) and…

You operate a single sequential worker that runs jobs one at a time, with no preemption and no idle gaps…

Implement a tiny search-query engine. Given a query string and a list of documents, parse the query into a…

You operate a fleet of microservices wired together by a dependency graph. The graph is given as an adjacency…

You run the spend planner for an ad platform. Given a total budget and a set of campaigns, you must split the…

When a customer switches plans partway through a billing cycle, the billing system must credit the unused…

You are given a list of raw inventory movements recorded across a network of warehouses. Each movement is an…

You are given a project broken into numTasks tasks, numbered 0 through numTasks - 1. Each task i takes…

You are concatenating several documents into one combined file. Each document carries an outline (a list of…

Build a deterministic simulator for a fixed-capacity in-memory cache that uses least-recently-used (LRU)…