Generated by Codex with GPT-5

Difficulty: MEDIUM
Problem: Reverse Words in a String

Problem gist

The input is a string that contains words separated by spaces. The output should contain the same words in reverse order, with exactly one space between adjacent words and no leading or trailing spaces.

The important detail is that the function is not reversing letters inside each word. It is reversing the sequence of words. For example, the words ["the", "sky", "is", "blue"] become ["blue", "is", "sky", "the"].

Generated by Codex with GPT-5

Difficulty: MEDIUM
Problem: Lowest Common Ancestor of a Binary Tree

Problem gist

Given the root of a binary tree and two existing nodes p and q, return their lowest common ancestor.

An ancestor can be the node itself. That detail matters: if p is above q, then p is the answer. “Lowest” means deepest in the tree, not smallest by value. This is a normal binary tree, so there is no useful ordering rule like there would be in a binary search tree.

Generated by Codex with gpt-5

Selected problem: URL Shortener Analytics

Scope: Design the analytics subsystem for a URL shortener: capture click events, compute useful aggregates, and serve owner dashboards without slowing down redirects.

Source grounding: Grokking frames analytics as an extended URL shortener requirement and calls out the danger of updating a shared counter row on every popular redirect; Alex Xu highlights that redirect choice affects analytics fidelity; DDIA supplies the storage and processing lens for event logs, OLTP versus OLAP separation, materialized aggregates, stream windows, partitioning, replication, and idempotent recovery.

Generated by Codex with GPT-5

Quick facts

• Difficulty: HARD
• Problem: Word Ladder
• Main tags: Breadth-First Search, Hash Table, String, Graph

What the problem is really asking

The input gives a beginWord, an endWord, and a dictionary called wordList.

Each move can change exactly one letter, and every intermediate word must appear in wordList. The task is to return the number of words in the shortest valid transformation sequence from beginWord to endWord. If no such sequence exists, return 0.

Generated by Codex with GPT-5

Quick facts

• Difficulty: MEDIUM
• Problem: Insert Delete GetRandom O(1)
• Main tags: Array, Hash Table, Math, Design, Randomized

What the problem is really asking

The task is to design a set-like data structure with three operations:

• insert(val) adds a value only if it is not already present.
• remove(val) deletes a value only if it is present.
• getRandom() returns one currently stored value, with every stored value equally likely.

The important constraint is that all three operations should run in average O(1) time.

Generated by Codex with gpt-5

Selected problem: Distributed Queue

Scope: Design a durable multi-tenant queue service for asynchronous jobs, service decoupling, retries, and fan-out-style workflows, with at-least-once delivery as the default and per-key ordering where callers explicitly need it.

Problem framing

This is the classic “design a distributed message queue” interview problem. Grokking’s interview process is the right starting point: clarify whether the interviewer wants a task queue, a pub/sub broker, or a retained event log before debating Kafka, SQS, or RabbitMQ. Alex Xu’s system design framing treats a message queue as the durability and decoupling layer between producers and consumers, and his notification, news feed, web crawler, chat, and video chapters show the same pattern repeatedly: put slow, unreliable, or bursty work behind a queue so producers and consumers can scale independently. DDIA adds the critical storage lesson: a queue is not just an in-memory list. It is a data system with ordering, replication, durability, retention, acknowledgements, and replay tradeoffs.

Generated by Codex with GPT-5

Quick facts

• Difficulty: HARD
• Problem: Basic Calculator
• Main tags: Math, String, Stack, Recursion

What the problem is really asking

The input is a string expression that contains non-negative integers, +, -, parentheses, and spaces. The task is to return the final numeric value without using Python’s built-in expression evaluator.

Generated by Codex with GPT-5

Quick facts

• Difficulty: MEDIUM
• Problem: Min Stack
• Main tags: Stack, Design

What the problem is really asking

The API looks simple: push, pop, top, and getMin.

The real requirement is hidden in getMin: it must return the current minimum in constant time, not by scanning the whole stack.

Generated by Codex with gpt-5

Selected problem: Payment System

Scope: Design a card-first online payment platform that creates payment intents, authorizes and captures funds, issues refunds, maintains an auditable ledger, and reconciles asynchronous processor outcomes without double-charging customers.

Problem framing

This is the classic “design a Stripe / Adyen / PSP-style payment platform for merchant checkout” interview problem. Grokking’s interview flow still applies directly: define the API contract early, size the system before arguing about storage, and then walk through the write path, read path, partitioning, caching, and failure handling. Alex Xu’s usual system design patterns also fit well here: stateless API servers on the edge, durable primary storage for transactional state, caches for read-heavy metadata, and queues or streams for slow or retryable side effects. DDIA adds the most important payment-specific lesson: integrity matters more than timeliness. A payment status can be briefly stale, but money must not be created, lost, or charged twice.

Generated by Codex with GPT-5

Quick facts

• Difficulty: MEDIUM
• Problem: Binary Tree Level Order Traversal
• Main tags: Tree, Breadth-First Search, Binary Tree

What the problem is really asking

The task is to read a binary tree one layer at a time.

Instead of visiting nodes in a depth-first order such as root, left subtree, right subtree, the problem wants the tree grouped by depth: