#Practice

Low Level Design

Design an Amazon Locker System

A complete low-level design walkthrough for Amazon Locker — from requirements clarification to token-based access, compartment management, and expiry enforcement in Java.

Easy·Sep 18, 2026·12 min read

Low Level Design OOPs SOLID

DSA

Task Scheduler

Three approaches — Math formula O(n), Greedy Max-Heap O(n log k), Sorted Array O(n). Find minimum CPU intervals with cooldown constraint.

Medium·Sep 16, 2026

DSA Heap Priority Queue 🏢 Amazon

DSA

Sort Characters By Frequency

Max-heap on character frequency. Time O(n + d log d), Space O(d) where d = distinct characters.

Medium·Sep 16, 2026

DSA Heap Priority Queue 🏢 Amazon

DSA

K Closest Points to Origin

Max-heap of size k — evict the farthest point on each insert. Time O(n log k), Space O(k).

Medium·Sep 16, 2026

DSA Heap Priority Queue 🏢 Amazon

DSA

Max Area of Island

DFS flood-fill to find the largest connected island. Time O(m×n), Space O(m×n).

Medium·Sep 16, 2026

DSA Graphs BFS 🏢 Amazon

DSA

Peak Index in a Mountain Array

Binary Search on the mountain's slope direction. O(log n) time, O(1) space.

Medium·Sep 16, 2026

DSA Binary Search LeetCode 🏢 Amazon

DSA

Majority Element II

Extended Boyer-Moore Voting — find all elements appearing more than ⌊n/3⌋ times. Time O(n), Space O(1).

Medium·Sep 15, 2026

DSA Arrays LeetCode 🏢 Amazon

DSA

Odd Even Linked List

In-place pointer weaving — odd indices first, then even. Optimal — Time O(n), Space O(1).

Medium·Sep 15, 2026

DSA Linked List LeetCode 🏢 Amazon

DSA

Path Sum

Check if a root-to-leaf path sums to a target. Optimal — Time O(n), Space O(h).

Easy·Sep 12, 2026

DSA LeetCode Practice

DSA

Remove Duplicates from Sorted Array II

Keep each unique element at most twice in-place. Optimal — Time O(n), Space O(1).

Medium·Sep 12, 2026

DSA Arrays LeetCode 🏢 Amazon

DSA

Generate Parentheses

Generate all combinations of well-formed parentheses using backtracking. Optimal — Time O(4ⁿ/√n), Space O(n).

Medium·Sep 11, 2026·6 min read

DSA Strings Recursion 🏢 Amazon

DSA

Rotate List

Covers: Naive, Optimal (circular link). Optimal — Time O(n), Space O(1).

Medium·Sep 11, 2026

DSA Linked List LeetCode 🏢 Amazon

DSA

Zigzag Conversion

Simulate row-by-row traversal with direction flip. Optimal — Time O(n), Space O(n).

Medium·Sep 9, 2026

DSA Strings LeetCode 🏢 Amazon

DSA

Rotate Array

Covers: Brute Force, Extra Array, Optimal (Reverse). Optimal — Time O(n), Space O(1).

Medium·Aug 29, 2026

DSA Arrays LeetCode

DSA

Contiguous Array

Covers: Brute Force, Optimal Prefix Sum. Best — Time O(n), Space O(n).

Medium·Aug 25, 2026

DSA Arrays LeetCode 🏢 Amazon

Low Level Design

Design a Conference Room Booking System

Low-level design of a thread-safe office room booking system featuring Singleton, Facade, Strategy (pluggable room selection), Observer (Email/Calendar/Slack notifications), Builder for recurring bookings, and atomic rollback.

Hard·Aug 21, 2026·20 min read

Low Level Design OOP Design Patterns

Low Level Design

Design Splitwise

Low-level design of an expense-sharing app — user and group management, three split strategies (equal, percentage, exact), Factory pattern for entity creation, Singleton orchestration, and a Facade that hides all internal complexity.

Hard·Aug 21, 2026·18 min read

Low Level Design OOP Design Patterns

Low Level Design

Design a Library Management System

A full low-level design for a library — rack-based book storage, dual borrow flows (by book ID or copy ID), multi-attribute search, borrow-log tracking, and a thin facade that separates the public API from the core engine.

Medium·Aug 16, 2026·18 min read

Low Level Design OOP Design Patterns

Low Level Design

Design a Notification Service

A multi-channel notification broker with per-service subscription control, parallel fan-out via an ExecutorService, exponential-backoff retry, and a Dead Letter Queue — built around Template Method, Strategy, Builder, and Singleton.

Medium·Aug 16, 2026·16 min read

Low Level Design OOP Design Patterns

Low Level Design

Design a Pub-Sub Model

Build a Publisher-Subscriber messaging system from scratch — decoupled components, dynamic topic management, and thread-safe broadcast using the Observer pattern and Singleton registry.

Medium·Aug 16, 2026·12 min read

Low Level Design OOP Design Patterns

Low Level Design

Design a Tic-Tac-Toe Game

A clean OOP design for the classic two-player grid game — pluggable winning strategies, a Singleton game controller, interface-driven board and game contracts, and a turn-based loop in Java.

Medium·Aug 16, 2026·14 min read

Low Level Design OOP Design Patterns

Low Level Design

Design a Logging Framework

A complete low-level design walkthrough for a custom logging framework — from requirements clarification to Singleton, Strategy, and Builder patterns, with async log processing and pluggable appenders in Java.

Easy·Aug 13, 2026·15 min read

Low Level Design OOP Design Patterns

Low Level Design

Design an LRU Cache

Build a fixed-capacity cache with O(1) get and put using a HashMap and a doubly linked list, with a pluggable eviction engine that cleanly separates storage from access-order tracking.

Medium·Aug 13, 2026·12 min read

Low Level Design OOP Design Patterns

Low Level Design

Design a Task Management System

A complete low-level design walkthrough for a task management system — from requirements to State, Strategy, Builder, and Singleton patterns with thread-safe task lifecycle management in Java.

Easy·Aug 13, 2026·15 min read

Low Level Design OOP Design Patterns

Low Level Design

Design a Traffic Signal Controller

A complete low-level design walkthrough for a traffic signal control system — State pattern for signal transitions, Singleton context as the FSM, and sensor-driven adaptive durations with emergency override.

Easy·Aug 11, 2026·12 min read

Low Level Design OOP Design Patterns

Low Level Design

Design a Bank Account System

A complete low-level design walkthrough for a bank account system — from requirements clarification to SOLID principles, design patterns, and a clean Java implementation with Singleton service, abstract accounts, and transaction history.

Easy·Aug 11, 2026·15 min read

Low Level Design OOP Design Patterns

Low Level Design

Design a Coffee Vending Machine

A complete low-level design walkthrough for a coffee vending machine — Singleton services, Factory for coffee types, synchronized thread safety, and inventory tracking with low-stock alerts.

Easy·Aug 11, 2026·15 min read

Low Level Design OOP Design Patterns

Concurrency

Blocking Queue — Producer-Consumer

Implement a thread-safe bounded blocking queue from scratch using ReentrantLock and Condition variables — the canonical producer-consumer concurrency problem.

Aug 10, 2026

Concurrency Java Multithreading

Concurrency

Building H2O — Semaphores + CyclicBarrier

Coordinate hydrogen and oxygen threads so they always form complete H2O molecules — never a stray atom — using two semaphores to enforce stoichiometry and a CyclicBarrier to synchronize the triplet.

Aug 10, 2026

Concurrency Java Multithreading

Concurrency

Dining Philosophers — Deadlock-Free Fork Ordering

Five philosophers, five forks, one circular table — eliminate the circular wait that causes deadlock by acquiring forks in a fixed global index order, never left-before-right.

Aug 10, 2026

Concurrency Java Deadlock

Concurrency

Print FooBar Alternately — Two Approaches

Two threads must alternate printing 'foo' and 'bar' exactly n times each. Compare a synchronized wait/notify approach against a cleaner paired-semaphore solution.

Aug 10, 2026

Concurrency Java Multithreading

Concurrency

Print In Order — CountDownLatch vs wait/notify

Three threads must print 'first', 'second', 'third' in order, regardless of scheduling. Compare CountDownLatch (clean one-shot signaling) against synchronized wait/notify (general state machine).

Aug 10, 2026

Concurrency Java Multithreading

Concurrency

Round Robin Print — N Threads in Order

N threads must print a shared counter in strict round-robin order: thread 0 prints 1, thread 1 prints 2, …, wrapping back to thread 0, until maxCount is reached.

Aug 10, 2026

Concurrency Java Multithreading

Concurrency

Print Zero-Odd-Even — Three-Thread Sequencing

Three threads must interleave to print '0102030405…' — zero before every number, odd and even alternating. Solved with ReentrantLock and three dedicated Condition variables.

Aug 10, 2026

Concurrency Java Multithreading

DSA

Implement Queue using Stacks

Stacks & Queues. Space O(n).

Easy·Aug 8, 2026

DSA Stack Queue

DSA

Next Greater Element II

Solution - Monotonic Stack approach. Optimal — Time O(2n), Space O(n).

Design an Amazon Locker System

Task Scheduler

Sort Characters By Frequency

K Closest Points to Origin

Max Area of Island

Peak Index in a Mountain Array

Majority Element II

Odd Even Linked List

Path Sum

Remove Duplicates from Sorted Array II

Generate Parentheses

Rotate List

Zigzag Conversion

Rotate Array

Contiguous Array

Design a Conference Room Booking System

Design Splitwise

Design a Library Management System

Design a Notification Service

Design a Pub-Sub Model

Design a Tic-Tac-Toe Game

Design a Logging Framework

Design an LRU Cache

Design a Task Management System

Design a Traffic Signal Controller

Design a Bank Account System

Design a Coffee Vending Machine

Blocking Queue — Producer-Consumer

Building H2O — Semaphores + CyclicBarrier

Dining Philosophers — Deadlock-Free Fork Ordering

Print FooBar Alternately — Two Approaches

Print In Order — CountDownLatch vs wait/notify

Round Robin Print — N Threads in Order

Print Zero-Odd-Even — Three-Thread Sequencing

Implement Queue using Stacks

Next Greater Element II

Largest Rectangle in a Histogram

Daily Temperature

Online Stock Span

Implement Min Stack

Sliding Window Maximum

Maximum Rectangle in Histogram

LFU Cache

Balanced Parentheses

Next Greater Element I

Implement Stack using Queues

Queue Using Array

Stack Using Array

Maximum Average Subarray I

Longest Substring Without Repeating Characters

Longest Subarray of 1's After Deleting One Element

Longest Repeating Character Replacement

Minimum Add to Make Parentheses Valid

String to Integer (atoi)

Capacity To Ship Packages Within D Days

Find First and Last Position of Element in Sorted Array

Encode and Decode a String

Decode String

Palindrome Number

Compare Version Numbers

Count And Say

Valid Anagram

Longest Common Prefix

Reverse Linked List

Roman to Int or vice versa

Longest Palindromic Substring

Reverse Words in a String

Valid Parentheses

Trapping Rainwater

Evaluate Reverse Polish Notation

Number of NGEs

Next Smaller Element

Linked List Cycle

Letter Combinations of a Phone Number

Insert Delete GetRandom O(1)

The Celebrity Problem

Reorder List

Merge k Sorted Lists

Middle Node