Hello Interview

Dated Sep 6, 2026; last modified on Sun, 06 Sep 2026

System Design

See for bite-sized groups that are digestible in a week’s period.

In a Hurry

Introduction
How to Prepare
Delivery Framework
Core Concepts
Key Technologies
Common Patterns
Question Breakdowns

Core Concepts

Networking Essentials
API Design
Data Modeling
Database Indexing
Caching
Sharding
Consistent Hashing
CAP Theorem
Numbers to know

Question Breakdowns

Patterns

Real-time Updates
Dealing with Contention
Multi-step Processes
Scaling Reads
Scaling Writes
Handling Large Blobs
Managing Long Running Tasks

Key Technologies

Redis
Elasticsearch
Kafka
API Gateway
Cassandra
DynamoDB
PostgreSQL
Flink
Zookeeper

Advanced Topics

Proximity Search
Data Structures for Big Data
Vector Databases
Time Series Databases
Change Data Capture

In the Wild

How Shopify Moved Inventory Reservations from Redis to MySQL
How Discord Moved Trillions of Messages to ScyllaDB
How Slack Put Kafka in Front of Its Redis Job Queue
How Figma Built Multiplayer Editing on Simplified CRDTs
How Spotify Serves Point Queries from an Exabyte Data Lake

Code

Two Pointers

Overview
Container With Most Water
Two Sum (Sorted Array)
3-Sum
Triangle Numbers
Move Zeroes
Sort Colors
Trapping Rain Water

Sliding Window

Overview
Maximum Sum of Subarrays of Size \(k\)
Max Points You Can Obtain from Cards
Max Sum of Distinct Subarrays of Length \(k\)
Overview
Longest Substring Without Repeating Characters
Longest Repeating Character Replacement

Intervals

Overview
Can Attend Meetings
Insert Interval
Non-Overlapping Intervals
Merge Intervals
Employee Free Time

Stack

Overview
Valid Parentheses
Decode String
Longest Valid Parentheses
Monotonic Stack
Daily Temperatures
Largest Rectangle in Histogram

Linked List

Overview
Linked List Cycle
Palindrome Linked List
Remove Nth Node from End of List
Reorder List
Swap Nodes in Pairs

Binary Search

Overview
Apple Harvest (Koko Eating Bananas)
Search in a Rotated Sorted Array
Split Array Largest Sum
Kth Smallest Element in a Sorted Matrix
Minimum Shipping Capacity

Heap

Overview
Kth Largest Element in an Array
K Closest Points to Origin
Find K Closest Elements
Merge K Sorted Lists
Median from Data Stream

Depth-First Search

Introduction
Fundamentals
Return Values
Maximum Depth of Binary Tree
Path Sum
Passing Values Down and Helper Functions
Validate Binary Search Tree
Calculate Tilt
Diameter of a Binary Tree
Path Sum II
Longest Univalue Path
Graphs Overview
Adjacency List
Copy Graph
Graph Valid Tree
Matrices
Flood Fill
Number of Islands
Surrounded Regions
Pacific Atlantic Water Flow

Breadth-First Search

Introduction
Overview
Level Order Sum
Rightmost Node
Zigzag Level Order
Maximum Width of Binary Tree
Graphs Overview
Minimum Knight Moves
Rotting Oranges
01-Matrix
Bus Routes

Backtracking

Overview
Word Search
Solution Space Trees
Subsets
Generate Parentheses
Combination Sum
Palindrome Partitioning
N-Queens

Graphs

Overview
Course Schedule
Course Schedule II
Shortest Path Algorithms
Network Delay Time
Cheapest Flights Within K Stops
Path with Minimum Effort
Find City with Fewest Reachable

Dynamic Programming

Fundamentals
Solving a Question with Dynamic Programming
Counting Bits
Decode Ways
Unique Paths
Maximal Square
Longest Increasing Subsequence
Word Break
Maximum Profit in Job Scheduling
Paint House
Paint House II
Minimum Window Subsequence

Greedy Algorithms

Overview
Best Time to Buy and Sell Stock
Gas Station
Jump Game
Jump Game II
Partition Labels

Trie

Overview
Implement Trie Methods
Prefix Matching

Prefix Sum

Overview
Count Vowels in Substrings
Subarray Sum Equals \(K\)

Matrices

Spiral Matrix
Rotate Image
Set Matrix Zeroes

Low Level Design

Low Level Design in a Hurry

Introduction
Delivery Framework
Design Principles
OOP Concepts
Design Patterns

Concurrency

Introduction
Correctness
Coordination
Scarcity

Problem Breakdowns

Connect Four
Amazon Locker
Elevator
Parking Lot
File System
Movie Ticket Booking
Logging Service
Rate Limiter
Inventory Management

Behavioral

Why the Behavioral Matters
Decode: How Interviews Work
Select: Choosing Responses
Deliver: Telling a Good Story
The Big Three Questions
Adapting to Big Tech
Practicing
Common Pitfalls
Special Interview Types
Answering AI Questions

AI Coding

Overview

Introduction
Interview Formats
Patterns
How to Prepare

Fundamentals

Codebase Orientation
Planning Your Approach
Driving the AI
Verification & Testing
Communication

Common Patterns

Graph Search & Pathfinding
Topological Sort
Backtracking
Greedy & Bin Packing
Dynamic Programming
String Matching and Parsing
Data Structure Design

AI Coding Problem Breakdowns

Battleship
Inventory Packer
Spell Checker
Card Game
Friend Recommender
Maze Solver
Route Planner
Task Scheduler
Word Container
Connect Four
Kitchen Orders
Maximize Unique Characters
Nonogram Solver

ML System Design

ML System Design in a Hurry

Introduction
Delivery Framework

ML System Design Core Concepts

Feature Engineering
Embeddings
Generalization
Evaluation

ML System Design Question Breakdowns

Harmful Content
Bot Detection
Video Recommendations

References

System Design Course | Hello Interview. www.hellointerview.com . Accessed Sep 6, 2026.
Data Structures and Algorithms Introduction | Hello Interview. www.hellointerview.com . Accessed Sep 6, 2026.