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Practice › LeetCode › LeetCode 38xx ›

LeetCode 3885 - Design Event Manager

LeetCode Problem 3885 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Heap (Priority Queue), Ordered Set Solution Problem Understanding The problem asks us to design an EventManager class that manages a set of events, each identified by a unique eventId and associated with a priority . The manager must support two primary operations: updating the priority of an active event and retrieving the highest-priority active event, with ties broken by...

Practice › LeetCode › LeetCode 38xx ›

LeetCode 3829 - Design Ride Sharing System

LeetCode Problem 3829 Difficulty: 🟡 Medium Topics: Hash Table, Design, Queue, Data Stream Solution Problem Understanding The problem asks us to design a ride sharing system that manages both riders and drivers as they enter the system, and to match them in a first-come, first-served order. The system must track riders who are waiting for rides, drivers who are available, and support canceling rider requests. Inputs include rider and driver...

Practice › LeetCode › LeetCode 38xx ›

LeetCode 3822 - Design Order Management System

LeetCode Problem 3822 Difficulty: 🟡 Medium Topics: Hash Table, Design Solution LeetCode 3822 - Design Order Management System Problem Understanding This problem asks us to implement a small in-memory order management system that supports four operations on trading orders. Each order has three attributes: orderId , a unique identifier orderType , either "buy" or "sell" price , an integer value When an order is added, it becomes active. Orders remain...

Practice › LeetCode › LeetCode 38xx ›

LeetCode 3815 - Design Auction System

LeetCode Problem 3815 Difficulty: 🟡 Medium Topics: Hash Table, Design, Heap (Priority Queue), Ordered Set Solution LeetCode 3815 - Design Auction System Problem Understanding This problem asks us to design a real time auction system that supports inserting, updating, removing, and querying bids efficiently. Each bid is uniquely identified by the pair (userId, itemId) . A user may have at most one active bid for a particular item. If addBid...

Practice › LeetCode › LeetCode 37xx ›

LeetCode 3709 - Design Exam Scores Tracker

LeetCode Problem 3709 Difficulty: 🟡 Medium Topics: Array, Binary Search, Design, Prefix Sum Solution Problem Understanding This problem asks us to design a system that tracks Alice's exam scores over time and efficiently computes the total score for any given time interval. The system must support two operations: recording a new exam with a timestamp and score, and querying the total score of exams between two timestamps inclusive. The input...

Practice › LeetCode › LeetCode 35xx ›

LeetCode 3508 - Implement Router

LeetCode Problem 3508 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Design, Queue, Ordered Set Solution Problem Understanding The problem is asking us to design a Router data structure to manage network packets efficiently under memory constraints. Each packet has three attributes: source , destination , and timestamp . The router has a memory limit , meaning it can store only a fixed number of packets. If a new...

Practice › LeetCode › LeetCode 34xx ›

LeetCode 3484 - Design Spreadsheet

LeetCode Problem 3484 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design, Matrix Solution LeetCode 3484 - Design Spreadsheet Problem Understanding This problem asks us to design a very small spreadsheet system that supports three operations on cells and one operation for evaluating simple formulas. The spreadsheet always contains exactly 26 columns, labeled 'A' through 'Z' , and a configurable number of rows. Each cell can store an integer value....

Practice › LeetCode › LeetCode 34xx ›

LeetCode 3408 - Design Task Manager

LeetCode Problem 3408 Difficulty: 🟡 Medium Topics: Hash Table, Design, Heap (Priority Queue), Ordered Set Solution LeetCode 3408. Design Task Manager Problem Understanding This problem asks us to design a data structure that maintains a collection of tasks belonging to different users. Each task has three pieces of information: userId , the user who owns the task taskId , a unique identifier for the task priority , which determines execution...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2080 - Range Frequency Queries

LeetCode Problem 2080 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Design, Segment Tree Solution Problem Understanding The problem asks us to design a data structure that can efficiently answer frequency queries on subarrays. We are given an integer array arr , and we need to support repeated queries of the form: Given left , right , and value Return how many times value appears in the subarray arr[left...right]...

Practice › LeetCode › LeetCode 32xx ›

LeetCode 3242 - Design Neighbor Sum Service

LeetCode Problem 3242 Difficulty: 🟢 Easy Topics: Array, Hash Table, Design, Matrix, Simulation Solution Problem Understanding The problem asks us to design a service that works on a square matrix, where every value in the matrix is unique. For any given value, we must efficiently compute two different types of neighbor sums. The first operation, adjacentSum(value) , returns the sum of all directly adjacent cells. These are the four cardinal...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1865 - Finding Pairs With a Certain Sum

LeetCode Problem 1865 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design Solution Problem Understanding This problem asks us to design a mutable data structure that supports two operations efficiently over two arrays, nums1 and nums2 . The first array, nums1 , is fixed after initialization and never changes. The second array, nums2 , is mutable, meaning its elements can be updated through add(index, val) operations. The goal of the data...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2069 - Walking Robot Simulation II

LeetCode Problem 2069 Difficulty: 🟡 Medium Topics: Design, Simulation Solution Problem Understanding The problem describes a simulation of a robot moving on a rectangular grid defined by width x height . The robot starts at the bottom-left corner (0, 0) facing East and moves in discrete steps. Each step moves the robot forward in its current direction unless the next cell is out of bounds, in which case the robot...

Practice › LeetCode › LeetCode 21xx ›

LeetCode 2166 - Design Bitset

LeetCode Problem 2166 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design Solution Problem Understanding This problem asks us to design a custom Bitset data structure that supports several operations efficiently. A bitset is simply a sequence of binary values, where each position stores either 0 or 1 . We are given a fixed size during initialization, and initially every bit is set to 0 . After that, the class...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2296 - Design a Text Editor

LeetCode Problem 2296 Difficulty: 🔴 Hard Topics: Linked List, String, Stack, Design, Simulation, Doubly-Linked List Solution Problem Understanding This problem asks us to design a simplified text editor that supports four operations: Inserting text at the current cursor position Deleting characters to the left of the cursor Moving the cursor left Moving the cursor right The cursor behaves exactly like a cursor in a normal text editor. It always sits...

Practice › LeetCode › LeetCode 23xx ›

LeetCode 2349 - Design a Number Container System

LeetCode Problem 2349 Difficulty: 🟡 Medium Topics: Hash Table, Design, Heap (Priority Queue), Ordered Set Solution LeetCode 2349 - Design a Number Container System Problem Understanding The problem asks us to design a data structure that supports two operations efficiently: Assign a number to a given index. Find the smallest index currently assigned to a given number. The important detail is that the change(index, number) operation can both insert and...

Practice › LeetCode › LeetCode 25xx ›

LeetCode 2590 - Design a Todo List

LeetCode Problem 2590 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design, Sorting Solution Problem Understanding This problem asks us to design a small task management system, similar to a lightweight todo application. We need to implement a TodoList class that supports adding tasks, marking tasks as completed, retrieving all pending tasks for a user, and filtering pending tasks by tag. Each task belongs to a specific user and contains...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2227 - Encrypt and Decrypt Strings

LeetCode Problem 2227 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Design, Trie Solution Problem Understanding This problem asks us to implement an Encrypter class that can both encrypt and decrypt strings according to custom character mappings. The keys array provides the characters that can be encrypted, and values provides the corresponding 2-character strings that each key maps to. The encryption of a string is straightforward: each character is replaced...

Practice › LeetCode › LeetCode 21xx ›

LeetCode 2102 - Sequentially Ordinal Rank Tracker

LeetCode Problem 2102 Difficulty: 🔴 Hard Topics: Design, Heap (Priority Queue), Data Stream, Ordered Set Solution Problem Understanding The problem asks us to design a data structure that continuously tracks scenic locations ranked by two rules: A higher score means a better location. If two locations have the same score, the lexicographically smaller name is considered better. The system supports two operations: add(name, score) inserts a new location. get() returns...

Practice › LeetCode › LeetCode 19xx ›

LeetCode 1912 - Design Movie Rental System

LeetCode Problem 1912 Difficulty: 🔴 Hard Topics: Array, Hash Table, Design, Heap (Priority Queue), Ordered Set Solution Problem Understanding The problem requires designing a movie rental system for multiple shops, where each shop carries at most one copy of each movie. We are asked to implement a class that efficiently supports searching for available movies, renting them, returning them, and reporting the cheapest currently rented movies. The input consists of...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1804 - Implement Trie II (Prefix Tree)

LeetCode Problem 1804 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Trie Solution Problem Understanding This problem asks us to implement a Trie data structure, also known as a prefix tree , which is designed to store strings efficiently in a way that allows fast lookups and prefix queries. We are required to implement the following operations: insert(word) : Add a word to the trie. countWordsEqualTo(word) : Count how many...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2276 - Count Integers in Intervals

LeetCode Problem 2276 Difficulty: 🔴 Hard Topics: Design, Segment Tree, Ordered Set Solution Problem Understanding This problem asks us to design a data structure that dynamically maintains a collection of integer intervals and efficiently reports how many distinct integers are covered by at least one interval. Initially, the interval set is empty. We must support two operations: add(left, right) , which inserts a new interval [left, right] count() , which...

Practice › LeetCode › LeetCode 24xx ›

LeetCode 2408 - Design SQL

LeetCode Problem 2408 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design Solution Problem Understanding This problem asks us to design a lightweight in memory database system that supports four core operations on multiple tables: Insert a row into a table Remove a row by id Select a single cell value Export all rows in CSV style format Each table has a fixed number of columns. The constructor receives two...

Practice › LeetCode › LeetCode 23xx ›

LeetCode 2353 - Design a Food Rating System

LeetCode Problem 2353 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design, Heap (Priority Queue), Ordered Set Solution Problem Understanding The problem is asking us to implement a food rating system that supports dynamic updates to the ratings of individual food items and allows querying for the highest-rated food for a given cuisine. Each food has a unique name, belongs to exactly one cuisine, and has an associated rating. The...

Practice › LeetCode › LeetCode 25xx ›

LeetCode 2502 - Design Memory Allocator

LeetCode Problem 2502 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Simulation Solution Problem Understanding The problem requires designing a memory allocator that manages a fixed-size memory array. Initially, all memory units are free, and the allocator supports two operations: allocating a block of consecutive free memory units for a given ID ( mID ) and freeing all memory units associated with a given mID . Allocation should always select...

Practice › LeetCode › LeetCode 26xx ›

LeetCode 2671 - Frequency Tracker

LeetCode Problem 2671 Difficulty: 🟡 Medium Topics: Hash Table, Design Solution Problem Understanding The problem asks us to design a data structure, FrequencyTracker , that can efficiently track the frequency of numbers and answer queries about whether any number exists with a specific frequency. The operations are add , which increases the count of a number; deleteOne , which decreases the count of a number if it exists; and hasFrequency...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2286 - Booking Concert Tickets in Groups

LeetCode Problem 2286 Difficulty: 🔴 Hard Topics: Binary Search, Design, Binary Indexed Tree, Segment Tree Solution Problem Understanding The problem asks us to design a ticket reservation system for a concert hall. The hall contains n rows, and each row contains exactly m seats. Seats in every row are numbered from left to right starting at 0 . We must support two different operations: The first operation, gather(k, maxRow) ,...

Practice › LeetCode › LeetCode 25xx ›

LeetCode 2526 - Find Consecutive Integers from a Data Stream

LeetCode Problem 2526 Difficulty: 🟡 Medium Topics: Hash Table, Design, Queue, Counting, Data Stream Solution Problem Understanding The problem asks us to design a data structure that continuously processes a stream of integers and determines whether the most recent k integers are all equal to a specific target value. We are given two parameters when initializing the data structure: value , which represents the integer we are interested in tracking....

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2254 - Design Video Sharing Platform

LeetCode Problem 2254 Difficulty: 🔴 Hard Topics: Hash Table, Design, Heap (Priority Queue) Solution Certainly. Here is a complete, detailed technical solution guide for LeetCode 2254 - Design Video Sharing Platform , following your requested formatting and style. Problem Understanding The problem asks us to design a class to simulate a video sharing platform. Each video is represented as a string of digits, where each digit corresponds to the content...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2241 - Design an ATM Machine

LeetCode Problem 2241 Difficulty: 🟡 Medium Topics: Array, Greedy, Design Solution Problem Understanding The problem asks us to design a simulation of an ATM machine that can handle deposits and withdrawals with strict rules about how money is dispensed. The ATM stores five types of banknotes: $20, $50, $100, $200, and $500. The key challenge is implementing the withdrawal logic, which prioritizes giving out larger denominations first. The deposit function...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2043 - Simple Bank System

LeetCode Problem 2043 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Simulation Solution Problem Understanding This problem asks us to design a very simple banking system that supports three operations: Transfer money between two accounts Deposit money into an account Withdraw money from an account The bank contains n accounts numbered from 1 to n . The initial balance for each account is given in a 0-indexed array named balance...

Practice › LeetCode › LeetCode 26xx ›

LeetCode 2642 - Design Graph With Shortest Path Calculator

LeetCode Problem 2642 Difficulty: 🔴 Hard Topics: Graph Theory, Design, Heap (Priority Queue), Shortest Path Solution Problem Understanding The problem asks us to design a graph data structure that supports two operations efficiently: Dynamically adding directed weighted edges. Finding the shortest path cost between two nodes. We are given a directed weighted graph with n nodes labeled from 0 to n - 1 . The graph initially contains a list...

Practice › LeetCode › LeetCode 33xx ›

LeetCode 3391 - Design a 3D Binary Matrix with Efficient Layer Tracking

LeetCode Problem 3391 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Heap (Priority Queue), Matrix, Ordered Set Solution Problem Understanding The problem asks us to design a data structure that manages a 3D binary matrix of size n x n x n . Every cell initially contains 0 , and we must support three operations efficiently: setCell(x, y, z) sets a specific cell to 1 . unsetCell(x, y, z) sets...

Practice › LeetCode › LeetCode 19xx ›

LeetCode 1993 - Operations on Tree

LeetCode Problem 1993 Difficulty: 🟡 Medium Topics: Array, Hash Table, Tree, Depth-First Search, Breadth-First Search, Design Solution LeetCode 1993 - Operations on Tree Problem Understanding The problem asks us to design a mutable tree-based data structure that supports three operations: lock , unlock , and upgrade . Each node in the tree may either be unlocked or locked by exactly one user. The operations must follow strict rules about when...

Practice › LeetCode › LeetCode 24xx ›

LeetCode 2424 - Longest Uploaded Prefix

LeetCode Problem 2424 Difficulty: 🟡 Medium Topics: Hash Table, Binary Search, Union-Find, Design, Binary Indexed Tree, Segment Tree, Heap (Priority Queue), Ordered Set Solution Problem Understanding The problem asks us to design a data structure that tracks uploaded videos and continuously reports the length of the longest uploaded prefix. A prefix of uploaded videos means that every video from 1 through i has already been uploaded. The goal is to...

Practice › LeetCode › LeetCode 33xx ›

LeetCode 3369 - Design an Array Statistics Tracker

LeetCode Problem 3369 Difficulty: 🔴 Hard Topics: Hash Table, Binary Search, Design, Queue, Heap (Priority Queue), Data Stream, Ordered Set Solution Problem Understanding The problem asks us to design a data structure that supports a stream of operations on a dynamic collection of integers. Numbers are added over time, and the oldest inserted number can also be removed. At any point, we must efficiently answer three statistical queries: The floored...

Practice › LeetCode › LeetCode 23xx ›

LeetCode 2336 - Smallest Number in Infinite Set

LeetCode Problem 2336 Difficulty: 🟡 Medium Topics: Hash Table, Design, Heap (Priority Queue), Ordered Set Solution Problem Understanding The problem asks us to implement a data structure representing an infinite set of positive integers, initially containing all integers starting from 1. We need to provide two main operations: popSmallest , which removes and returns the smallest integer in the set, and addBack , which adds a previously removed integer back...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1348 - Tweet Counts Per Frequency

LeetCode Problem 1348 Difficulty: 🟡 Medium Topics: Hash Table, String, Binary Search, Design, Sorting, Ordered Set Solution Problem Understanding The problem is asking us to design a system that tracks tweets by their timestamp and allows querying the number of tweets in specific time intervals, split according to a frequency (minute, hour, or day). Specifically, we need to implement a TweetCounts class with two operations: recording a tweet and retrieving...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1476 - Subrectangle Queries

LeetCode Problem 1476 Difficulty: 🟡 Medium Topics: Array, Design, Matrix Solution Problem Understanding The problem asks us to design a class called SubrectangleQueries that operates on a two dimensional integer matrix, referred to as a rectangle. This class must support two operations efficiently: The first operation, updateSubrectangle(row1, col1, row2, col2, newValue) , modifies a rectangular region inside the matrix. Every element whose coordinates fall within the specified top left corner...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 535 - Encode and Decode TinyURL

LeetCode Problem 535 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Hash Function Solution Problem Understanding The problem asks us to implement a URL shortening service, similar to TinyURL. Given a long URL, our system should generate a short, unique URL that maps back to the original URL. When the short URL is accessed, the system should return the original long URL. Essentially, we need a reversible mapping between long...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1586 - Binary Search Tree Iterator II

LeetCode Problem 1586 Difficulty: 🟡 Medium Topics: Stack, Tree, Design, Binary Search Tree, Binary Tree, Iterator Solution Problem Understanding The problem requires implementing a BSTIterator class that allows forward and backward traversal over the in-order sequence of a Binary Search Tree (BST). In-order traversal of a BST visits nodes in ascending order. The iterator must support four operations: next() , prev() , hasNext() , and hasPrev() . Initially, the iterator...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 604 - Design Compressed String Iterator

LeetCode Problem 604 Difficulty: 🟢 Easy Topics: Array, String, Design, Iterator Solution Problem Understanding This problem asks us to implement a compressed string iterator . We are given a string where each character is immediately followed by a number representing how many times that character appears consecutively in the uncompressed version. For example, "a3b2" represents "aaabb" . We are required to implement a class StringIterator with two methods: next() ,...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1825 - Finding MK Average

LeetCode Problem 1825 Difficulty: 🔴 Hard Topics: Design, Queue, Heap (Priority Queue), Data Stream, Ordered Set Solution Problem Understanding The problem asks us to design a data structure that continuously receives numbers from a stream and can efficiently compute a special average called the MKAverage. For every query, we only care about the most recent m elements in the stream. From those m elements: Remove the smallest k numbers. Remove...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 588 - Design In-Memory File System

LeetCode Problem 588 Difficulty: 🔴 Hard Topics: Hash Table, String, Design, Trie, Sorting Solution Problem Understanding The problem asks us to design an in-memory file system that simulates basic file system operations without interacting with the real filesystem. We need to implement a FileSystem class that supports creating directories, adding and reading file content, and listing files or directories in a given path. The input consists of paths, which are...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 155 - Min Stack

LeetCode Problem 155 Difficulty: 🟡 Medium Topics: Stack, Design Solution Problem Understanding The problem asks us to design a custom stack data structure that behaves like a normal stack, while also supporting an additional operation called getMin() . This operation must return the minimum value currently stored in the stack, and it must do so in constant time, O(1) . A normal stack already supports these standard operations efficiently: push(val)...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 716 - Max Stack

LeetCode Problem 716 Difficulty: 🔴 Hard Topics: Linked List, Stack, Design, Doubly-Linked List, Ordered Set Solution Problem Understanding The problem asks us to design a stack data structure, MaxStack , which behaves like a normal stack with additional operations for efficiently accessing and removing the maximum element. Specifically, in addition to the standard stack operations push , pop , and top , we need to implement peekMax , which returns...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 676 - Implement Magic Dictionary

LeetCode Problem 676 Difficulty: 🟡 Medium Topics: Hash Table, String, Depth-First Search, Design, Trie Solution Problem Understanding The problem asks us to design a dictionary-like data structure that supports two operations. First, we load a collection of distinct words into the structure. Second, given a query string, we determine whether it is possible to modify exactly one character in the query so that the resulting string matches one of the...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1656 - Design an Ordered Stream

LeetCode Problem 1656 Difficulty: 🟢 Easy Topics: Array, Hash Table, Design, Data Stream Solution Problem Understanding This problem asks us to simulate a data stream where each element arrives with a unique ID in arbitrary order. Each element consists of an integer idKey (between 1 and n ) and a string value . The goal is to design a stream that, after every insertion, returns the largest contiguous chunk of...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 146 - LRU Cache

LeetCode Problem 146 Difficulty: 🟡 Medium Topics: Hash Table, Linked List, Design, Doubly-Linked List Solution Problem Understanding The problem asks us to design an efficient cache that follows the Least Recently Used, or LRU, eviction policy. A cache stores a limited number of key-value pairs. When the cache reaches its maximum capacity and a new item must be inserted, the least recently used item should be removed first. The class...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 307 - Range Sum Query - Mutable

LeetCode Problem 307 Difficulty: 🟡 Medium Topics: Array, Divide and Conquer, Design, Binary Indexed Tree, Segment Tree Solution Problem Understanding The problem asks us to design a mutable range sum data structure. We are given an integer array nums , and we must efficiently support two operations: Updating the value at a specific index. Querying the sum of elements within a range [left, right] . The important detail is that...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 288 - Unique Word Abbreviation

LeetCode Problem 288 Difficulty: 🟡 Medium Topics: Array, Hash Table, String, Design Solution Problem Understanding The problem asks us to design a data structure that determines whether a word's abbreviation is unique within a given dictionary. An abbreviation is defined using three parts: The first character The number of characters between the first and last character The last character For example: "dog" becomes "d1g" "internationalization" becomes "i18n" "it" remains "it"...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1172 - Dinner Plate Stacks

LeetCode Problem 1172 Difficulty: 🔴 Hard Topics: Hash Table, Stack, Design, Heap (Priority Queue) Solution Problem Understanding The problem asks us to design a data structure that behaves like an infinite row of stacks, where every stack has the same fixed capacity. Instead of working with a single stack, we maintain many stacks indexed from left to right starting at 0 . The operations are constrained by specific rules: When...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1032 - Stream of Characters

LeetCode Problem 1032 Difficulty: 🔴 Hard Topics: Array, String, Design, Trie, Data Stream Solution Problem Understanding The problem asks us to design a data structure that processes a stream of characters one at a time. After each newly added character, we must determine whether any suffix of the stream matches one of the words from a predefined dictionary. A suffix means the ending portion of the stream. For example, if...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 705 - Design HashSet

LeetCode Problem 705 Difficulty: 🟢 Easy Topics: Array, Hash Table, Linked List, Design, Hash Function Solution Problem Understanding The problem asks us to design our own HashSet implementation without using any built in hash table libraries such as Python's set or Go's built in map type as the primary solution idea. A HashSet is a data structure that stores unique values. It supports three operations: add(key) , insert a value...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1396 - Design Underground System

LeetCode Problem 1396 Difficulty: 🟡 Medium Topics: Hash Table, String, Design Solution Problem Understanding This problem asks us to design a small tracking system for an underground railway network. The system must support three operations: Recording when a customer checks into a station. Recording when a customer checks out from a station. Calculating the average travel time between two stations. Each customer is identified by a unique integer id ....

Practice › LeetCode › LeetCode 03xx ›

LeetCode 303 - Range Sum Query - Immutable

LeetCode Problem 303 Difficulty: 🟢 Easy Topics: Array, Design, Prefix Sum Solution Problem Understanding The problem asks us to design a data structure that supports efficient range sum queries on a fixed array. We are given an integer array nums , and after initialization, the array never changes. The goal is to answer multiple queries of the form: What is the sum of all elements between index left and index...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 281 - Zigzag Iterator

LeetCode Problem 281 Difficulty: 🟡 Medium Topics: Array, Design, Queue, Iterator Solution Problem Understanding The problem asks us to design an iterator that alternates between two input arrays, returning elements in a cyclic or zigzag order. Instead of consuming all elements from the first vector and then moving to the second vector, we must switch back and forth between them whenever possible. For example, if the input vectors are [1,...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 384 - Shuffle an Array

LeetCode Problem 384 Difficulty: 🟡 Medium Topics: Array, Math, Design, Randomized Solution Problem Understanding The problem asks us to design a data structure that supports two operations on an integer array: Reset the array back to its original order. Return a randomly shuffled version of the array. The important requirement is that every possible permutation of the array must be equally likely. This means the shuffle operation cannot introduce statistical...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 211 - Design Add and Search Words Data Structure

LeetCode Problem 211 Difficulty: 🟡 Medium Topics: String, Depth-First Search, Design, Trie Solution Problem Understanding The problem asks us to design a data structure that supports two operations efficiently: Adding words into the structure. Searching for words, where the search pattern may contain the wildcard character '.' . The wildcard '.' represents any single lowercase English letter. This means that a search query is not always an exact string match....

Practice › LeetCode › LeetCode 06xx ›

LeetCode 677 - Map Sum Pairs

LeetCode Problem 677 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Trie Solution Problem Understanding The problem asks us to design a custom data structure that behaves like a map from strings to integers, while also supporting efficient prefix-based sum queries. There are two operations: insert(key, val) This operation stores a string key with an integer value. If the key already exists, its old value must be replaced with the...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1157 - Online Majority Element In Subarray

LeetCode Problem 1157 Difficulty: 🔴 Hard Topics: Array, Binary Search, Design, Binary Indexed Tree, Segment Tree Solution Problem Understanding The problem asks us to design a data structure that can efficiently answer majority element queries on subarrays of a given array. Specifically, given an array arr and multiple queries of the form (left, right, threshold) , the goal is to find an element within the subarray arr[left...right] that occurs at...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 173 - Binary Search Tree Iterator

LeetCode Problem 173 Difficulty: 🟡 Medium Topics: Stack, Tree, Design, Binary Search Tree, Binary Tree, Iterator Solution It looks like your prompt accidentally concatenated two problems. I will answer the second one, LeetCode 173 - Binary Search Tree Iterator , following your required format. Problem Understanding This problem asks us to design an iterator for a Binary Search Tree (BST) that returns values in in-order traversal order . In a...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1261 - Find Elements in a Contaminated Binary Tree

LeetCode Problem 1261 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Breadth-First Search, Design, Binary Tree Solution Problem Understanding The problem gives us a binary tree where every node value has been contaminated and replaced with -1 . However, we know the exact rule that originally generated the values in the tree. The original tree followed these rules: The root node always had value 0 If a node had...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1357 - Apply Discount Every n Orders

LeetCode Problem 1357 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design Solution Problem Understanding The problem describes a supermarket that sells products identified by unique integer IDs. Each product has a corresponding price. Customers purchase products in certain amounts, generating a bill that is the sum of the prices multiplied by the amounts purchased. The supermarket offers a periodic discount: every nth customer gets a percentage discount off their subtotal....

Practice › LeetCode › LeetCode 09xx ›

LeetCode 901 - Online Stock Span

LeetCode Problem 901 Difficulty: 🟡 Medium Topics: Stack, Design, Monotonic Stack, Data Stream Solution Problem Understanding The problem asks us to design a class that processes stock prices one day at a time and, for every new price, returns the stock span for that day. The stock span is defined as the number of consecutive days ending today where the stock price was less than or equal to today's price....

Practice › LeetCode › LeetCode 06xx ›

LeetCode 635 - Design Log Storage System

LeetCode Problem 635 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Ordered Set Solution Problem Understanding The problem is asking us to design a log storage system that supports two primary operations: storing logs with unique IDs and timestamps, and retrieving logs based on a timestamp range and granularity. Each timestamp is represented as a string in the format YYYY:MM:DD:HH:MM:SS , which is zero-padded and guarantees lexicographic ordering aligns with...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 353 - Design Snake Game

LeetCode Problem 353 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Queue, Simulation Solution Problem Understanding The problem asks us to design a mutable data structure that simulates the classic Snake game. The game is played on a rectangular grid with dimensions height x width . The snake starts at the top-left corner (0, 0) and initially has length 1 . The game receives a predefined sequence of food positions....

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1603 - Design Parking System

LeetCode Problem 1603 Difficulty: 🟢 Easy Topics: Design, Simulation, Counting Solution Problem Understanding This problem asks us to design a very small parking lot management system. The parking lot contains exactly three types of parking spaces: Big parking spaces Medium parking spaces Small parking spaces The constructor receives the number of available spaces for each type. After initialization, cars arrive one at a time, and for every arriving car we...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1244 - Design A Leaderboard

LeetCode Problem 1244 Difficulty: 🟡 Medium Topics: Hash Table, Design, Sorting Solution LeetCode 1244 - Design A Leaderboard Problem Understanding This problem asks us to design a leaderboard system that supports three operations efficiently. Each player has a unique playerId and an associated score. The leaderboard begins empty, and players can be added dynamically as operations are performed. The addScore(playerId, score) operation either creates a new player with the given...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 352 - Data Stream as Disjoint Intervals

LeetCode Problem 352 Difficulty: 🔴 Hard Topics: Hash Table, Binary Search, Union-Find, Design, Data Stream, Ordered Set Solution LeetCode 352 - Data Stream as Disjoint Intervals Problem Understanding This problem asks us to process a stream of non-negative integers and continuously maintain a compact representation of all numbers seen so far. Instead of storing every individual number separately, we want to group consecutive numbers into disjoint intervals. An interval [start,...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 284 - Peeking Iterator

LeetCode Problem 284 Difficulty: 🟡 Medium Topics: Array, Design, Iterator Solution Problem Understanding The problem asks us to extend the functionality of a standard iterator with an additional peek operation. A standard iterator provides two operations: next() , which returns the next element in the sequence and advances the pointer, and hasNext() , which tells whether there are more elements left. The PeekingIterator enhances this interface with peek() , which...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 933 - Number of Recent Calls

LeetCode Problem 933 Difficulty: 🟢 Easy Topics: Design, Queue, Data Stream Solution Problem Understanding The problem asks us to implement a class RecentCounter that tracks the number of requests (or pings) made in the last 3000 milliseconds. Every time the ping(t) method is called with a time t , we are asked to return how many requests occurred within the time interval [t - 3000, t] . The class starts...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1600 - Throne Inheritance

LeetCode Problem 1600 Difficulty: 🟡 Medium Topics: Hash Table, Tree, Depth-First Search, Design Solution Problem Understanding The problem asks us to design a data structure that simulates a royal inheritance system. There is a king at the top of the family tree, and over time people can be born or die. We must support three operations efficiently: Add a child to an existing person. Mark a person as dead. Return...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 855 - Exam Room

LeetCode Problem 855 Difficulty: 🟡 Medium Topics: Design, Heap (Priority Queue), Ordered Set Solution Problem Understanding This problem requires designing a simulation for an exam room seating arrangement. We have n seats in a single row, labeled from 0 to n - 1 . Students enter one by one, and each student chooses a seat such that the distance to the closest occupied seat is maximized. If there are multiple...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1483 - Kth Ancestor of a Tree Node

LeetCode Problem 1483 Difficulty: 🔴 Hard Topics: Binary Search, Dynamic Programming, Bit Manipulation, Tree, Depth-First Search, Breadth-First Search, Design Solution Problem Understanding The problem gives us a rooted tree with n nodes labeled from 0 to n - 1 . Instead of providing the tree as adjacency lists or edges, the input is represented using a parent array. For every node i , parent[i] tells us which node is its...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1472 - Design Browser History

LeetCode Problem 1472 Difficulty: 🟡 Medium Topics: Array, Linked List, Stack, Design, Doubly-Linked List, Data Stream Solution Problem Understanding The problem is asking us to simulate a single-tab browser with a history mechanism. You start on a homepage , and from there, you can visit new URLs, back track a certain number of steps, or forward a certain number of steps. The inputs consist of method calls: BrowserHistory(homepage) initializes the...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 622 - Design Circular Queue

LeetCode Problem 622 Difficulty: 🟡 Medium Topics: Array, Linked List, Design, Queue Solution Problem Understanding The problem asks us to implement a circular queue, a fixed-size queue where the end wraps back to the start to efficiently reuse empty space. In a normal queue implemented with an array, once the array fills, even if elements are dequeued from the front, new elements cannot be enqueued without shifting the array. A...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 703 - Kth Largest Element in a Stream

LeetCode Problem 703 Difficulty: 🟢 Easy Topics: Tree, Design, Binary Search Tree, Heap (Priority Queue), Binary Tree, Data Stream Solution Problem Understanding The problem asks us to design a data structure that continuously tracks the kth largest element in a stream of integers. We are given an integer k and an initial list of numbers called nums . After initialization, new numbers are added one at a time using the...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 297 - Serialize and Deserialize Binary Tree

LeetCode Problem 297 Difficulty: 🔴 Hard Topics: String, Tree, Depth-First Search, Breadth-First Search, Design, Binary Tree Solution Problem Understanding The problem asks us to design two complementary operations for a binary tree: serialization and deserialization. Serialization converts a binary tree into a string representation that can be stored or transmitted. Deserialization performs the reverse operation, reconstructing the exact same binary tree structure from the serialized string. The important detail is...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 900 - RLE Iterator

LeetCode Problem 900 Difficulty: 🟡 Medium Topics: Array, Design, Counting, Iterator Solution Problem Understanding This problem asks us to design an iterator over a run-length encoded sequence instead of storing the full sequence explicitly. In run-length encoding, the array is organized in pairs: encoding[i] represents how many times a value appears encoding[i + 1] represents the value itself For example: [3,8,2,5] means: 8 appears 3 times 5 appears 2 times...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 346 - Moving Average from Data Stream

LeetCode Problem 346 Difficulty: 🟢 Easy Topics: Array, Design, Queue, Data Stream Solution Problem Understanding The problem asks us to design a data structure that continuously receives integers from a stream and returns the average of the most recent values within a fixed-size sliding window. A sliding window means that we only care about the latest size elements. As new values arrive, older values eventually fall out of the window...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 432 - All O`one Data Structure

LeetCode Problem 432 Difficulty: 🔴 Hard Topics: Hash Table, Linked List, Design, Doubly-Linked List Solution Problem Understanding The problem asks us to design a special data structure that stores string keys together with their occurrence counts. The structure must support incrementing and decrementing counts, while also being able to instantly retrieve one key with the maximum count and one key with the minimum count. Unlike ordinary frequency counting problems, this...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 729 - My Calendar I

LeetCode Problem 729 Difficulty: 🟡 Medium Topics: Array, Binary Search, Design, Segment Tree, Ordered Set Solution Problem Understanding The problem asks us to design a calendar system that supports booking events without allowing overlapping intervals. Each event is represented as a half-open interval [startTime, endTime) . This means the event includes startTime , but does not include endTime . For example, the interval [10, 20) contains all times from 10...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 707 - Design Linked List

LeetCode Problem 707 Difficulty: 🟡 Medium Topics: Linked List, Design Solution LeetCode 707 - Design Linked List Problem Understanding This problem asks us to implement our own linked list data structure from scratch without using any built in linked list library. The linked list must support several common operations such as retrieving a value at a specific index, inserting nodes at different positions, and deleting nodes. The list is 0-indexed,...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 745 - Prefix and Suffix Search

LeetCode Problem 745 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Design, Trie Solution Problem Understanding The problem asks us to design a data structure that supports efficient searches based on both a prefix and a suffix. We are given an array of words, where each word has an implicit index based on its position in the array. The WordFilter class must support two operations: Initialization with the list of...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1628 - Design an Expression Tree With Evaluate Function

LeetCode Problem 1628 Difficulty: 🟡 Medium Topics: Array, Math, Stack, Tree, Design, Binary Tree Solution Problem Understanding The problem requires constructing a binary expression tree from a postfix arithmetic expression and implementing a method to evaluate the tree. The input is an array of strings postfix representing numbers and operators ( + , - , * , / ) in postfix notation , meaning operands appear before their operators. The...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 271 - Encode and Decode Strings

LeetCode Problem 271 Difficulty: 🟡 Medium Topics: Array, String, Design Solution Problem Understanding The problem asks us to design a reversible encoding system for a list of strings. We need two functions: encode , which converts a list of strings into a single string decode , which reconstructs the original list from that encoded string The important requirement is that the transformation must be lossless. After encoding and then decoding,...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1570 - Dot Product of Two Sparse Vectors

LeetCode Problem 1570 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, Design Solution Problem Understanding This problem asks us to efficiently compute the dot product between two sparse vectors. A normal vector is simply an array of numbers. The dot product of two vectors is computed by multiplying corresponding elements and summing the results. For two vectors a and b of the same length: $$a \cdot b = \sum_{i=0}^{n-1}...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 348 - Design Tic-Tac-Toe

LeetCode Problem 348 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Matrix, Simulation Solution Problem Understanding The problem asks us to design a data structure that simulates an n x n Tic-Tac-Toe game between two players. We need to implement a class that supports two operations: Initializing a game board of size n Processing moves one at a time and immediately determining whether the current move causes a player to...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 380 - Insert Delete GetRandom O(1)

LeetCode Problem 380 Difficulty: 🟡 Medium Topics: Array, Hash Table, Math, Design, Randomized Solution Problem Understanding This problem asks us to design a custom data structure called RandomizedSet that supports three operations: Insert a value into the set. Remove a value from the set. Return a random element from the set. The critical requirement is that all three operations must run in average O(1) time complexity. The behavior is very...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 379 - Design Phone Directory

LeetCode Problem 379 Difficulty: 🟡 Medium Topics: Array, Hash Table, Linked List, Design, Queue Solution Problem Understanding The problem asks us to design a data structure that manages a fixed pool of phone numbers. Initially, every number from 0 to maxNumbers - 1 is available. The directory must support three operations efficiently. The get() operation should assign and return an available number. If no numbers remain, it should return -1...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 225 - Implement Stack using Queues

LeetCode Problem 225 Difficulty: 🟢 Easy Topics: Stack, Design, Queue Solution Problem Understanding The problem asks us to implement a stack data structure while using only queue operations internally. A stack follows the Last-In-First-Out, or LIFO, principle. This means the most recently inserted element must be removed first. In contrast, a queue follows the First-In-First-Out, or FIFO, principle, where elements leave in the same order they entered. We must design...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 732 - My Calendar III

LeetCode Problem 732 Difficulty: 🔴 Hard Topics: Binary Search, Design, Segment Tree, Prefix Sum, Ordered Set Solution Problem Understanding This problem asks us to design a calendar system that supports adding time intervals and reporting the highest number of overlapping events seen so far. Each booking is represented as a half open interval [startTime, endTime) . This means the event includes startTime but excludes endTime . Two events overlap only...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 460 - LFU Cache

LeetCode Problem 460 Difficulty: 🔴 Hard Topics: Hash Table, Linked List, Design, Doubly-Linked List Solution Problem Understanding The problem asks us to design a cache that supports two operations: get(key) returns the value associated with a key if it exists, otherwise -1 put(key, value) inserts or updates a key-value pair Unlike a normal cache, eviction is not based only on recency. Instead, this cache uses the Least Frequently Used policy....

Practice › LeetCode › LeetCode 03xx ›

LeetCode 341 - Flatten Nested List Iterator

LeetCode Problem 341 Difficulty: 🟡 Medium Topics: Stack, Tree, Depth-First Search, Design, Queue, Iterator Solution Problem Understanding This problem asks us to design an iterator that can traverse a deeply nested list structure as if it were a flat list of integers. The input is not a normal array. Each element inside nestedList can be one of two things: A single integer Another list containing more NestedInteger objects Those nested...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2013 - Detect Squares

LeetCode Problem 2013 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Counting, Data Stream Solution Problem Understanding The problem presents a stream of points in a 2D plane and asks us to design a data structure that supports two operations efficiently: adding points and counting squares. The add operation inserts a point into the data structure, and the count operation calculates the number of axis-aligned squares that can be formed...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 244 - Shortest Word Distance II

LeetCode Problem 244 Difficulty: 🟡 Medium Topics: Array, Hash Table, Two Pointers, String, Design Solution LeetCode 244 - Shortest Word Distance II Problem Understanding This problem asks us to design a reusable data structure that can efficiently answer repeated shortest-distance queries between words in a fixed list of strings. We are given an array of words, wordsDict , during initialization. After that, we must repeatedly answer queries asking: What is...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 706 - Design HashMap

LeetCode Problem 706 Difficulty: 🟢 Easy Topics: Array, Hash Table, Linked List, Design, Hash Function Solution Problem Understanding The problem requires us to implement a HashMap from scratch without using any built-in hash table libraries. In other words, we need to create a data structure that can store key-value pairs, allow efficient insertion, retrieval, and deletion of values based on their keys. The input operations consist of put , get...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 308 - Range Sum Query 2D - Mutable

LeetCode Problem 308 Difficulty: 🟡 Medium Topics: Array, Design, Binary Indexed Tree, Segment Tree, Matrix Solution Problem Understanding The problem asks us to design a mutable 2D range sum data structure. We are given a matrix of integers, and we must support two operations efficiently: Update the value at a specific cell. Compute the sum of all values inside a rectangular submatrix. The rectangle is defined by two corners: Upper-left...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 295 - Find Median from Data Stream

LeetCode Problem 295 Difficulty: 🔴 Hard Topics: Two Pointers, Design, Sorting, Heap (Priority Queue), Data Stream Solution Problem Understanding The problem asks us to design a data structure that continuously receives integers from a stream and can efficiently return the median of all numbers seen so far. The median is defined as the middle element in a sorted list. If the number of elements is odd, the median is the...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 642 - Design Search Autocomplete System

LeetCode Problem 642 Difficulty: 🔴 Hard Topics: String, Depth-First Search, Design, Trie, Sorting, Heap (Priority Queue), Data Stream Solution Problem Understanding The problem asks us to design an autocomplete system that behaves similarly to a search engine suggestion feature. The system starts with a collection of historical sentences and the number of times each sentence has been typed before. As the user types characters one by one, the system must...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1670 - Design Front Middle Back Queue

LeetCode Problem 1670 Difficulty: 🟡 Medium Topics: Array, Linked List, Design, Queue, Doubly-Linked List, Data Stream Solution Problem Understanding The problem is asking us to design a specialized queue that allows insertions and removals not only at the front and back, but also at the middle of the queue. We need to maintain the queue such that we can efficiently access and modify elements at these three positions. The middle...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 911 - Online Election

LeetCode Problem 911 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Design Solution Problem Understanding The problem is asking us to design a data structure that can efficiently answer queries about the leader of an election at any given time. We are given two integer arrays, persons and times . The persons array represents the sequence of votes, where persons[i] is the candidate who received the i th vote....

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1352 - Product of the Last K Numbers

LeetCode Problem 1352 Difficulty: 🟡 Medium Topics: Array, Math, Design, Data Stream, Prefix Sum Solution Problem Understanding This problem asks us to design a data structure that continuously processes a stream of integers and supports querying the product of the last k inserted numbers. The ProductOfNumbers class starts with an empty sequence. Every time add(num) is called, a new integer is appended to the stream. Later, getProduct(k) must return the...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1166 - Design File System

LeetCode Problem 1166 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Trie Solution Problem Understanding This problem asks us to design a simplified file system that supports two operations: Creating a new path with an associated integer value. Retrieving the value stored at a path. A path behaves similarly to a directory structure in a real operating system. Every path is composed of components separated by / . For example:...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 355 - Design Twitter

LeetCode Problem 355 Difficulty: 🟡 Medium Topics: Hash Table, Linked List, Design, Heap (Priority Queue) Solution Problem Understanding This problem asks us to design a simplified version of a social media platform similar to Twitter. The system must support four main operations: Users can post tweets. Users can follow other users. Users can unfollow other users. Users can retrieve a personalized news feed containing the 10 most recent tweets from...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1381 - Design a Stack With Increment Operation

LeetCode Problem 1381 Difficulty: 🟡 Medium Topics: Array, Stack, Design Solution Problem Understanding This problem requires designing a specialized stack data structure, called CustomStack , that not only supports the usual push and pop operations but also an incremental operation that can increase the value of the bottom k elements by a given amount. In other words, this stack behaves like a standard stack for pushing and popping elements, but...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 919 - Complete Binary Tree Inserter

LeetCode Problem 919 Difficulty: 🟡 Medium Topics: Tree, Breadth-First Search, Design, Binary Tree Solution Problem Understanding This problem asks us to design a data structure that allows insertion into a complete binary tree while maintaining its completeness. A complete binary tree is one where all levels are fully filled, except possibly the last, which is filled from left to right. The key requirement is that after each insertion, the tree...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1500 - Design a File Sharing System

LeetCode Problem 1500 Difficulty: 🟡 Medium Topics: Hash Table, Design, Sorting, Heap (Priority Queue), Data Stream Solution Problem Understanding This problem asks us to design a miniature peer-to-peer file sharing system. The file is divided into m chunks, where every chunk has an ID from 1 to m . Users can join the system while already owning some chunks, leave the system entirely, and request chunks from other users. The...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 304 - Range Sum Query 2D - Immutable

LeetCode Problem 304 Difficulty: 🟡 Medium Topics: Array, Design, Matrix, Prefix Sum Solution Problem Understanding The problem asks us to design a data structure that can efficiently answer multiple rectangular sum queries on a fixed 2D matrix. We are given a matrix of integers, and each query specifies four coordinates: (row1, col1) represents the top-left corner of a rectangle (row2, col2) represents the bottom-right corner of the rectangle For every...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 715 - Range Module

LeetCode Problem 715 Difficulty: 🔴 Hard Topics: Design, Segment Tree, Ordered Set Solution Problem Understanding This problem asks us to design a data structure that dynamically tracks intervals on the number line. The tracked intervals are represented as half-open intervals, meaning [left, right) includes every value x such that left <= x < right . The data structure must support three operations: addRange(left, right) marks every value in [left, right)...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1756 - Design Most Recently Used Queue

LeetCode Problem 1756 Difficulty: 🟡 Medium Topics: Array, Linked List, Divide and Conquer, Design, Simulation, Doubly-Linked List Solution Problem Understanding The problem asks us to design a special queue called an "Most Recently Used Queue", abbreviated as MRUQueue. Initially, the queue contains the integers from 1 to n in increasing order. The key operation is fetch(k) , where k is 1-indexed. This operation does two things: It finds the kth...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1845 - Seat Reservation Manager

LeetCode Problem 1845 Difficulty: 🟡 Medium Topics: Design, Heap (Priority Queue) Solution Problem Understanding The problem requires designing a system to manage seat reservations for n seats numbered from 1 to n . You need to implement a SeatManager class with two main operations: reserve and unreserve . The reserve operation must always allocate the smallest-numbered available seat , while unreserve releases a previously reserved seat so it becomes available...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 362 - Design Hit Counter

LeetCode Problem 362 Difficulty: 🟡 Medium Topics: Array, Binary Search, Design, Queue, Data Stream Solution Problem Understanding The problem asks us to design a data structure that tracks how many events, called "hits", occurred during the last 5 minutes. Every hit comes with a timestamp measured in seconds, and timestamps are guaranteed to arrive in chronological order. The class must support three operations: hit(timestamp) records a new hit at the...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 170 - Two Sum III - Data structure design

LeetCode Problem 170 Difficulty: 🟢 Easy Topics: Array, Hash Table, Two Pointers, Design, Data Stream Solution Problem Understanding The problem asks us to design a data structure that supports two operations efficiently over a stream of integers. The first operation, add(number) , inserts a number into the data structure. Numbers may appear multiple times, so duplicates must be handled correctly. The second operation, find(value) , checks whether there exists any...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 981 - Time Based Key-Value Store

LeetCode Problem 981 Difficulty: 🟡 Medium Topics: Hash Table, String, Binary Search, Design Solution Problem Understanding The problem asks us to design a data structure that behaves like a key-value store, but with an important twist: the same key can have multiple values over time. Every value is associated with a timestamp, and later queries must return the most recent value that existed at or before a requested timestamp. In...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1286 - Iterator for Combination

LeetCode Problem 1286 Difficulty: 🟡 Medium Topics: String, Backtracking, Design, Iterator Solution Problem Understanding The problem asks us to design a class CombinationIterator that generates all combinations of a given length from a string of sorted, distinct lowercase letters. The iterator must return these combinations in lexicographical order , meaning alphabetically. The input consists of two components: a string characters , which contains unique letters sorted in ascending order, and...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 731 - My Calendar II

LeetCode Problem 731 Difficulty: 🟡 Medium Topics: Array, Binary Search, Design, Segment Tree, Prefix Sum, Ordered Set Solution Problem Understanding The problem asks us to design a calendar system that supports booking time intervals while enforcing one important rule: no point in time may be covered by three events simultaneously. Each event is represented as a half open interval [startTime, endTime) . This means the event includes startTime but excludes...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 449 - Serialize and Deserialize BST

LeetCode Problem 449 Difficulty: 🟡 Medium Topics: String, Tree, Depth-First Search, Breadth-First Search, Design, Binary Search Tree, Binary Tree Solution Problem Understanding The problem asks us to design two operations for a Binary Search Tree, abbreviated as BST: serialize(root) converts the BST into a string representation. deserialize(data) reconstructs the exact same BST from that string. The important requirement is that the deserialized tree must match the original tree structure and...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 381 - Insert Delete GetRandom O(1) - Duplicates allowed

LeetCode Problem 381 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Design, Randomized Solution Problem Understanding The problem asks us to design a data structure called RandomizedCollection that behaves like a multiset. Unlike a normal set, duplicate values are allowed. The data structure must support three operations: Insert a value Remove one occurrence of a value Return a random element The critical requirement is that every operation must run in...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1206 - Design Skiplist

LeetCode Problem 1206 Difficulty: 🔴 Hard Topics: Linked List, Design Solution Problem Understanding This problem asks us to implement a Skiplist from scratch, a probabilistic data structure that allows efficient search, insertion, and deletion operations. A skiplist consists of multiple levels of sorted linked lists, where each higher level acts as an "express lane" to skip over multiple elements in lower levels. The bottom-most level contains all elements in sorted...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1622 - Fancy Sequence

LeetCode Problem 1622 Difficulty: 🔴 Hard Topics: Math, Design, Segment Tree, Number Theory Solution Problem Understanding The problem asks us to design a mutable sequence data structure that supports four operations efficiently: Append a value to the sequence. Add a constant value to every element currently in the sequence. Multiply every element currently in the sequence by a constant. Retrieve the value at a specific index. All returned values must...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1146 - Snapshot Array

LeetCode Problem 1146 Difficulty: 🟡 Medium Topics: Array, Hash Table, Binary Search, Design Solution Problem Understanding The problem asks us to implement a SnapshotArray , a data structure that behaves like an array but also allows taking snapshots of its state at any moment and retrieving the value of any element at a specific snapshot. Specifically, the data structure needs to support three operations: set(index, val) , snap() , and...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1724 - Checking Existence of Edge Length Limited Paths II

LeetCode Problem 1724 Difficulty: 🔴 Hard Topics: Depth-First Search, Union-Find, Graph Theory, Design, Sorting, Heap (Priority Queue), Minimum Spanning Tree Solution LeetCode 1724 - Checking Existence of Edge Length Limited Paths II Problem Understanding This problem gives us an undirected weighted graph with n nodes and a list of edges. Each edge connects two nodes and has an associated distance value. The graph may contain multiple edges between the same...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1429 - First Unique Number

LeetCode Problem 1429 Difficulty: 🟡 Medium Topics: Array, Hash Table, Design, Queue, Data Stream Solution Problem Understanding The problem requires designing a data structure that can efficiently maintain and retrieve the first unique integer in a queue of numbers. You are given an initial list of integers, and after that, the data structure should support two operations efficiently: retrieving the first unique number ( showFirstUnique ) and adding a new...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1797 - Design Authentication Manager

LeetCode Problem 1797 Difficulty: 🟡 Medium Topics: Hash Table, Linked List, Design, Doubly-Linked List Solution Problem Understanding The problem asks us to design an authentication system that manages tokens with expiration times. Each token is valid for a fixed timeToLive seconds starting from the moment it is generated or renewed. The system must support three operations: generating a new token, renewing an existing unexpired token, and counting the number of...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 251 - Flatten 2D Vector

LeetCode Problem 251 Difficulty: 🟡 Medium Topics: Array, Two Pointers, Design, Iterator Solution Problem Understanding The problem asks us to design an iterator that traverses a two dimensional array as though it were a single flat sequence. Instead of returning nested lists, the iterator should expose elements one at a time through two methods, next() and hasNext() . The input is a 2D vector, meaning a list of lists. Each...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 359 - Logger Rate Limiter

LeetCode Problem 359 Difficulty: 🟢 Easy Topics: Hash Table, Design, Data Stream Solution Problem Understanding The problem asks us to design a logging system that controls how frequently identical messages can be printed. Each message is associated with a timestamp, and the same message is only allowed to be printed once every 10 seconds. More specifically, when a message is printed at timestamp t , the next time the exact...

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LeetCode 232 - Implement Queue using Stacks

LeetCode Problem 232 Difficulty: 🟢 Easy Topics: Stack, Design, Queue Solution Problem Understanding The problem asks us to implement a queue using only stack operations. A queue follows the FIFO, First In First Out, principle. The first element inserted into the queue must be the first element removed. In contrast, a stack follows the LIFO, Last In First Out, principle, where the most recently inserted element is removed first. We...

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LeetCode 631 - Design Excel Sum Formula

LeetCode Problem 631 Difficulty: 🔴 Hard Topics: Array, Hash Table, String, Graph Theory, Design, Topological Sort, Matrix Solution Problem Understanding The problem asks us to design a miniature spreadsheet system similar to Microsoft Excel. The spreadsheet contains cells arranged in rows and columns, where rows are numbered starting from 1 and columns are labeled using uppercase letters such as A , B , C , and so on. Each cell...

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LeetCode 641 - Design Circular Deque

LeetCode Problem 641 Difficulty: 🟡 Medium Topics: Array, Linked List, Design, Queue Solution Problem Understanding The problem asks us to design a circular double-ended queue, also called a deque. A deque is a data structure that allows insertion and deletion from both the front and the rear. Unlike a normal queue, which only supports enqueue at the back and dequeue at the front, a deque supports operations at both ends...

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LeetCode 895 - Maximum Frequency Stack

LeetCode Problem 895 Difficulty: 🔴 Hard Topics: Hash Table, Stack, Design, Ordered Set Solution Problem Understanding The problem is asking us to design a custom stack-like data structure that supports two operations: pushing values onto the stack and popping the most frequent element. A key difference from a traditional stack is that the pop operation does not necessarily remove the element at the top; instead, it removes the element with...

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LeetCode 2034 - Stock Price Fluctuation

LeetCode Problem 2034 Difficulty: 🟡 Medium Topics: Hash Table, Design, Heap (Priority Queue), Data Stream, Ordered Set Solution Here’s a complete, detailed technical solution guide for LeetCode 2034 - Stock Price Fluctuation following your formatting and content requirements. Problem Understanding The problem presents a stream of stock price records where each record consists of a timestamp and a price . However, the stream is not guaranteed to be chronological, and...

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LeetCode 431 - Encode N-ary Tree to Binary Tree

LeetCode Problem 431 Difficulty: 🔴 Hard Topics: Tree, Depth-First Search, Breadth-First Search, Design, Binary Tree Solution Problem Understanding This problem asks us to design a reversible transformation between two different tree structures: An N-ary tree, where each node can have any number of children A binary tree, where each node has at most two children The important requirement is not how the encoding looks internally, but that the process is...

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LeetCode 208 - Implement Trie (Prefix Tree)

LeetCode Problem 208 Difficulty: 🟡 Medium Topics: Hash Table, String, Design, Trie Solution Problem Understanding The problem asks us to design and implement a Trie, also called a Prefix Tree. A Trie is a specialized tree structure for storing strings in a way that makes prefix-based operations very efficient. We need to implement three operations: insert(word) stores a word inside the Trie. search(word) checks whether the exact word exists in...

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LeetCode 1032: Stream of Characters

Problem Restatement We implement a StreamChecker class: StreamChecker(words) : Initialize with a list of words. query(letter) : Add the letter to the stream. Return true if any word in words is a suffix of the letters queried so far. The official constraints state that 1 <= words.length <= 2000 , 1 <= words[i].length <= 2000 , and up to 4 * 10^4 calls to query . Input and Output Item...

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LeetCode 900: RLE Iterator

Problem Restatement We need to design an iterator over a run-length encoded sequence. The encoded array stores pairs: [count, value] At every even index i , encoding[i] is the number of times encoding[i + 1] appears in the decoded sequence. For example: encoding = [3, 8, 0, 9, 2, 5] represents: [8, 8, 8, 5, 5] because there are three 8 s, zero 9 s, and two 5 s. The...

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LeetCode 919: Complete Binary Tree Inserter

Problem Restatement We need to design a class called CBTInserter . It is initialized with the root of a complete binary tree. A complete binary tree has every level full except possibly the last level, and the last level is filled from left to right. The class must support: Method Meaning CBTInserter(root) Initialize with an existing complete binary tree insert(val) Insert a new node while keeping the tree complete, and...

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LeetCode 895: Maximum Frequency Stack

Problem Restatement Design a data structure called FreqStack . It must support two operations: Operation Meaning push(val) Push integer val onto the stack pop() Remove and return the value with highest frequency If multiple values have the same highest frequency, pop() must remove and return the one that was pushed most recently. So this is like a stack, but priority is based on: Higher frequency first. More recent push first...

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LeetCode 745: Prefix and Suffix Search

Problem Restatement We are given a list of words. We need to design a class: WordFilter that supports queries: f(prefix, suffix) The query should return the largest index of a word that: Starts with prefix Ends with suffix If no such word exists, return: -1 The official problem asks for the maximum index satisfying both prefix and suffix constraints. Input and Output Item Meaning Constructor WordFilter(words) Query f(pref, suff) Output...

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LeetCode 244: Shortest Word Distance II

Problem Restatement We need to design a class called WordDistance . The class receives a list of words in the constructor. Then it must support a method called shortest(word1, word2) . This method returns the shortest index distance between word1 and word2 in the original list. The important detail is that shortest will be called many times with different pairs of words. So we should avoid scanning the whole word...

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LeetCode 716: Max Stack

Problem Restatement We need to design a data structure called MaxStack . It behaves like a normal stack, but also supports operations related to the maximum value. The required operations are: Operation Meaning push(x) Push x onto the stack pop() Remove and return the top element top() Return the top element without removing it peekMax() Return the maximum element without removing it popMax() Remove and return the maximum element If...

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LeetCode 622: Design Circular Queue

Problem Restatement We need to design a circular queue. A queue follows FIFO order: first in, first out. A circular queue uses a fixed-size array, but when the end of the array is reached, the next position wraps around to the beginning. This lets the queue reuse empty space created by earlier removals. The class must support these operations: Operation Meaning MyCircularQueue(k) Create a queue with capacity k enQueue(value) Insert...

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LeetCode 715: Range Module

Problem Restatement We need to design a data structure called RangeModule . It tracks ranges of numbers. Each range is represented as a half-open interval: [left, right) This means the interval includes left , but does not include right . The data structure must support three operations: Operation Meaning addRange(left, right) Start tracking every number in [left, right) queryRange(left, right) Return whether every number in [left, right) is tracked removeRange(left,...

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LeetCode 911: Online Election

Problem Restatement We need to implement a class called TopVotedCandidate . We are given two arrays: persons times The i -th vote was cast for persons[i] at time times[i] . For a query time t , we need to return the candidate who was leading at time t . Votes cast exactly at time t count. If there is a tie, the candidate who received the most recent vote among...

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LeetCode 710: Random Pick with Blacklist

Problem Restatement We need to design a class that randomly returns an integer from the range: [0, n - 1] Some values are blacklisted and cannot be returned. Every allowed value must have the same probability. The class supports: Operation Meaning Solution(n, blacklist) Initialize the picker pick() Return a uniformly random allowed value The challenge is to make pick() efficient. Input and Output Item Meaning Input Integer n Input Array...

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LeetCode 642: Design Search Autocomplete System

Problem Restatement We need to design an autocomplete system. The system starts with historical sentences and their usage counts. For every typed character except # , return up to three historical sentences that start with the current prefix. The result must be sorted by: Priority Rule First Higher frequency first Second ASCII order if frequencies are equal When the user types # , the current sentence is finished. The system...

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LeetCode 933: Number of Recent Calls

Problem Restatement We need to implement a class called RecentCounter . It supports one main operation: ping(t) Each call records a request at time t , measured in milliseconds. After recording the request, ping(t) must return how many requests happened in the time range: [t - 3000, t] The current request is included. The problem guarantees that every new t is strictly larger than the previous one. The official statement...

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LeetCode 641: Design Circular Deque

Problem Restatement We need to design a circular double-ended queue. A deque supports insertion and deletion from both ends: Operation Meaning insertFront(value) Add a value to the front insertLast(value) Add a value to the rear deleteFront() Remove the front value deleteLast() Remove the rear value getFront() Read the front value getRear() Read the rear value The deque has a fixed maximum capacity k . If we try to insert into...

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LeetCode 707: Design Linked List

Problem Restatement We need to design a linked list from scratch. The implementation may use either a singly linked list or a doubly linked list. This guide uses a singly linked list. A singly linked list node stores two things: Field Meaning val The value stored in the node next A reference to the next node The linked list is 0-indexed. We need to implement the MyLinkedList class with these...

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LeetCode 635: Design Log Storage System

Problem Restatement We need to design a log storage system. Each log has: Field Meaning id Unique log ID timestamp Time string in Year:Month:Day:Hour:Minute:Second format The timestamp format looks like this: "2017:01:01:23:59:59" All parts are zero-padded. The system needs two operations: Operation Meaning put(id, timestamp) Store a log retrieve(start, end, granularity) Return IDs whose timestamps are inside the inclusive range The granularity tells us how much of the timestamp to...

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LeetCode 706: Design HashMap

Problem Restatement We need to design a class called MyHashMap . The class should support three operations: Operation Meaning put(key, value) Insert or update the value for key get(key) Return the value for key , or -1 if missing remove(key) Remove key and its value if it exists We cannot use built-in hash table libraries. A hash map stores key-value pairs. Each key maps to one value. If put is...

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LeetCode 732: My Calendar III

Problem Restatement We need to implement a calendar. Each event is a half-open interval: [start, end) A new event can always be added. After each booking, we must return the largest number of events that overlap at any time. This value is called the maximum k -booking. A k -booking means there is some non-empty time interval where k events are active at the same time. The official problem defines...

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LeetCode 981: Time Based Key-Value Store

Problem Restatement We need to design a data structure called TimeMap . It supports two operations: Operation Meaning set(key, value, timestamp) Store value for key at timestamp get(key, timestamp) Return the value for key with the largest stored timestamp <= timestamp If no stored timestamp is less than or equal to the query timestamp, return an empty string. The important guarantee is that, for each key, calls to set happen...

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LeetCode 705: Design HashSet

Problem Restatement We need to design a class called MyHashSet . The class should support three operations: Operation Meaning add(key) Insert key into the hash set remove(key) Remove key from the hash set if it exists contains(key) Return whether key exists in the hash set We cannot use built-in hash table libraries. The value range is bounded: 0 <= key <= 1_000_000 At most 10,000 calls will be made to...

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LeetCode 731: My Calendar II

Problem Restatement We need to implement a calendar that supports event bookings. Each booking is a half-open interval: [start, end) A booking is allowed if it does not create a triple booking. This means: One event overlapping another is allowed. Three events overlapping at the same time is forbidden. We must implement: book(start, end) Return: True if the event can be added. False if adding it would create a triple...

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LeetCode 855: Exam Room

Problem Restatement We need to design an ExamRoom class. There are n seats in one row, labeled: 0, 1, 2, ..., n - 1 When a student enters, they choose the seat that maximizes the distance to the closest occupied seat. If several seats give the same distance, the student chooses the smallest seat number. If the room is empty, the student sits at seat 0 . We need to...

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LeetCode 631: Design Excel Sum Formula

Problem Restatement We need to design a simplified Excel spreadsheet. The spreadsheet has: Item Meaning height Number of rows width Last column letter, from A to Z Initial value Every cell starts as 0 We need to support three operations: Operation Meaning set(row, column, val) Set a cell to a fixed integer value get(row, column) Return the current value of a cell sum(row, column, numbers) Set a cell to a...

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LeetCode 729: My Calendar I

Problem Restatement We need to implement a calendar. The calendar supports one operation: book(startTime, endTime) Each booking represents a half-open interval: [startTime, endTime) This means the event includes startTime , but does not include endTime . A new event can be added only if it does not create a double booking. A double booking happens when the new event overlaps with an existing event. If the event can be added,...

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LeetCode 703: Kth Largest Element in a Stream

Problem Restatement We need to design a class called KthLargest . The class receives: k nums nums is the initial stream of integers. After that, the class supports an operation: add(val) This adds val to the stream and returns the kth largest element among all numbers seen so far. The kth largest element means the element at position k if all numbers are sorted from largest to smallest. Duplicates count...

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LeetCode 604: Design Compressed String Iterator

Problem Restatement We need to design a class called StringIterator . The input is a compressed string. The compressed string has repeated groups of: letter + positive integer The integer tells us how many times that letter appears in the original uncompressed string. For example: "L1e2t1C1o1d1e1" represents: "LeetCode" because: Compressed Part Meaning L1 L appears once e2 e appears twice t1 t appears once C1 C appears once o1 o...

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LeetCode 588: Design In-Memory File System

Problem Restatement We need to design an in-memory file system. The file system must support four operations: Operation Behavior ls(path) If path is a file, return only that file name. If path is a directory, return its direct children in lexicographic order. mkdir(path) Create the directory path. If middle directories do not exist, create them too. addContentToFile(filePath, content) Create the file if missing, otherwise append content to the existing file....

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LeetCode 677: Map Sum Pairs

Problem Restatement We need to design a data structure called MapSum . It supports two operations: Operation Meaning insert(key, val) Store the value val for the string key sum(prefix) Return the total value of all keys that start with prefix If a key already exists, the new value replaces the old value. It does not add to the old value. For example: insert("apple", 3) sum("ap") # 3 insert("app", 2) sum("ap")...

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LeetCode 901: Online Stock Span

Problem Restatement We need to design a class called StockSpanner . The class receives stock prices one day at a time through the method next(price) . For each new price, we return its stock span. The span is the number of consecutive days ending today where each price was less than or equal to today's price. For example: prices = [100, 80, 60, 70, 60, 75, 85] The spans are:...

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LeetCode 676: Implement Magic Dictionary

Problem Restatement We need to design a data structure called MagicDictionary . It supports two operations: Operation Meaning buildDict(dictionary) Store a list of distinct words search(searchWord) Return whether searchWord can become a dictionary word by changing exactly one character The important rule is exactly one character must be changed. Zero changes do not count. Two or more changes do not count. The modified word must exist in the dictionary. For...

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LeetCode 535: Encode and Decode TinyURL

Problem Restatement We need to design a class that can shorten a long URL and later recover the original URL. The class has two methods: encode(longUrl) and: decode(shortUrl) encode receives a valid long URL and returns a tiny URL. decode receives a tiny URL generated by the same object and returns the original long URL. The problem does not restrict the algorithm. It only requires that a URL encoded by...

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LeetCode 449: Serialize and Deserialize BST

Problem Restatement We need to design two methods: Method Meaning serialize(root) Convert a BST into a string deserialize(data) Convert that string back into the original BST The tree is guaranteed to be a binary search tree. There is no required format. Any format is valid as long as deserializing the serialized string reconstructs the same BST. The encoded string should be as compact as possible. The number of nodes is...

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LeetCode 348: Design Tic-Tac-Toe

Problem Restatement We need to design a Tic-Tac-Toe game played by two players on an n x n board. The class supports two operations: Method Meaning TicTacToe(n) Initializes an n x n board move(row, col, player) Places player at position (row, col) and returns the winner The rules are: Rule Meaning Valid move Every move is guaranteed to be placed on an empty cell Game state No moves are made...

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LeetCode 297: Serialize and Deserialize Binary Tree

Problem Restatement We need to design a codec for a binary tree. The codec has two operations: serialize(root) deserialize(data) serialize(root) converts a binary tree into a string. deserialize(data) converts that string back into the original binary tree. There is no required string format. The only requirement is that a tree serialized by our method can be deserialized back into the same tree structure. The source statement also clarifies that we...

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LeetCode 225: Implement Stack using Queues

Problem Restatement We need to implement a stack using only queues. A stack follows LIFO order: last in, first out A queue follows FIFO order: first in, first out We need to implement the class MyStack with these operations: Method Meaning push(x) Push x onto the stack pop() Remove and return the top element top() Return the top element without removing it empty() Return whether the stack is empty The...

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LeetCode 346: Moving Average from Data Stream

Problem Restatement We need to design a class that calculates the moving average of the last size values in a data stream. The class supports: Method Meaning MovingAverage(size) Initializes the moving window size next(val) Adds a value and returns the current moving average The moving average is computed over the most recent values currently inside the window. If fewer than size values have appeared so far, average over all available...

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LeetCode 295: Find Median from Data Stream

Problem Restatement We need to design a class called MedianFinder . It supports two operations: addNum(num) findMedian() addNum(num) adds one integer from the data stream. findMedian() returns the median of all numbers added so far. The median is the middle value in a sorted list. If the list has an even number of values, the median is the average of the two middle values. The answer is accepted if it...

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LeetCode 271: Encode and Decode Strings

Problem Restatement We need to design two methods: encode(strs) decode(s) The encode method receives a list of strings and converts it into one string. The decode method receives that encoded string and reconstructs the original list. We cannot use serialization helpers such as eval . The strings may contain any of the 256 valid ASCII characters, including delimiters, digits, spaces, and empty strings. The constraints are 1 <= strs.length <=...

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LeetCode 288: Unique Word Abbreviation

Problem Restatement We are given a dictionary of words. We need to design a class that can answer this query: isUnique(word) It returns whether word has a unique abbreviation in the dictionary. The abbreviation rule is: If the word length is less than 3 , the abbreviation is the word itself. Otherwise: first letter + number of middle letters + last letter For example: "dog" -> "d1g" "internationalization" -> "i18n"...

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LeetCode 173: Binary Search Tree Iterator

Problem Restatement We need to implement a class: BSTIterator It represents an iterator over the inorder traversal of a binary search tree. For a BST, inorder traversal visits values in ascending order. The class must support: Method Meaning BSTIterator(root) Initialize the iterator with the root of the BST next() Move to the next value and return it hasNext() Return whether there is still a next value The pointer starts before...

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LeetCode 146: LRU Cache

Problem Restatement Design a data structure that supports: Operation Meaning get(key) Return the value if the key exists, otherwise -1 put(key, value) Insert or update the key-value pair The cache has a fixed capacity. When the cache exceeds capacity, remove the least recently used item. Both operations must run in: O(1) average time. LeetCode defines the least recently used item as the entry that has not been accessed or updated...

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LeetCode 170: Two Sum III - Data Structure Design

Problem Restatement We need to design a data structure that supports two operations: add(number) find(value) The operations behave like this: Operation Meaning add(number) Add the number into the internal data structure find(value) Return True if any pair of numbers sums to value The same number may be added multiple times. The pair must use two different occurrences unless the number appears at least twice. For example: add(1) add(3) add(5) Then:...

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LeetCode 211: Design Add and Search Words Data Structure

Problem Restatement We need to design a data structure that supports two operations: Operation Meaning addWord(word) Add a word into the data structure search(word) Return whether a word matches the query The search query may contain lowercase letters and the special character: . A dot matches any one lowercase letter. The official LeetCode problem asks us to implement WordDictionary with addWord and search , where search supports . as a...

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LeetCode 284: Peeking Iterator

Problem Restatement We are given an existing iterator with these operations: next() hasNext() We need to design a new iterator called PeekingIterator that also supports: peek() peek() should return the next element without moving the iterator forward. That means calling: peek() multiple times should always return the same value until: next() is called. The official statement defines peek() as retrieving the next element without advancing the iterator. The wrapped iterator...

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LeetCode 341: Flatten Nested List Iterator

Problem Restatement We are given a nested list of integers. Each element is either: Type Meaning Integer A single integer List A nested list whose elements may also be integers or lists We need to implement an iterator that returns the integers in flattened left-to-right order. The class must support: Method Meaning NestedIterator(nestedList) Initializes the iterator next() Returns the next integer hasNext() Returns true if there are more integers LeetCode...

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LeetCode 362: Design Hit Counter

Problem Restatement We need to design a hit counter. The counter supports two operations: Method Meaning hit(timestamp) Record one hit at timestamp getHits(timestamp) Return how many hits happened in the past 300 seconds The timestamp is given in seconds. Calls arrive in chronological order, so timestamps are monotonically increasing. Multiple hits may happen at the same timestamp. The time window is the past 5 minutes, which means the past 300...

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LeetCode 460: LFU Cache

Problem Restatement We need to design a Least Frequently Used cache. The cache supports two operations: get(key) put(key, value) The cache has a fixed capacity . If the cache is full and we need to insert a new key, we remove the least frequently used key. Each key has a use counter. A key's frequency increases when: get(key) is called on it. put(key, value) is called on an existing key....

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LeetCode 208: Implement Trie Prefix Tree

Problem Restatement We need to implement a Trie, also called a prefix tree. The Trie must support three operations: Operation Meaning insert(word) Add word into the Trie search(word) Return True if the exact word exists startsWith(prefix) Return True if any inserted word starts with prefix The official problem asks us to implement the Trie class with insert , search , and startsWith . The Trie stores lowercase English words and...

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LeetCode 359: Logger Rate Limiter

Problem Restatement We need to design a logger system. The logger receives messages with timestamps. Each message should be printed only if the same message was not printed in the last 10 seconds. Given: timestamp: int message: str Return: True if the message should be printed. Return: False if the message should be blocked. A message printed at timestamp t prevents the same message from being printed again until timestamp...

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LeetCode 281: Zigzag Iterator

Problem Restatement We are given two integer lists, v1 and v2 . We need to implement an iterator that returns their elements alternately. For example: v1 = [1, 2] v2 = [3, 4, 5, 6] The iterator should return: 1, 3, 2, 4, 5, 6 Once one list runs out, we continue returning elements from the other list. The class has three operations: ZigzagIterator(v1, v2) next() hasNext() The source statement...

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LeetCode 385: Mini Parser

Problem Restatement We are given a string s . The string represents either: A single integer A nested list of integers Examples: "324" "[123,[456,[789]]]" "[-1,[2,-3],4]" We need to parse this string and return a NestedInteger object. The NestedInteger class is already provided by LeetCode. We only need to implement: deserialize(s) Input and Output Item Meaning Input A serialized string s Output A NestedInteger object Integer format May be positive or...

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LeetCode 232: Implement Queue using Stacks

Problem Restatement We need to implement a queue using only standard stack operations. The queue must support: Operation Meaning push(x) Add x to the back of the queue pop() Remove and return the front element peek() Return the front element without removing it empty() Return whether the queue is empty A queue follows FIFO order: First In, First Out A stack follows LIFO order: Last In, First Out The challenge...

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LeetCode 308: Range Sum Query 2D - Mutable

Problem Restatement We are given a 2D integer matrix. We need to implement a class called NumMatrix that supports two operations: Operation Meaning update(row, col, val) Change matrix[row][col] to val sumRegion(row1, col1, row2, col2) Return the sum of all cells inside the rectangle The rectangle is inclusive. Unlike LeetCode 304, the matrix can change after construction. So a fixed 2D prefix sum matrix is no longer enough. Input and Output...

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LeetCode 384: Shuffle an Array

Problem Restatement We are given an integer array nums . We need to design a class with two operations: Operation Meaning reset() Return the array to its original configuration shuffle() Return a random permutation of the array The important requirement is that every possible permutation must be equally likely. For an array of length n , there are n! possible permutations. A correct shuffle gives each one the same probability....

Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

LeetCode 432: All O'one Data Structure

Problem Restatement We need to design a data structure that stores string counts. It must support these operations: Operation Meaning inc(key) Increase key count by 1 ; insert it with count 1 if missing dec(key) Decrease key count by 1 ; remove it if count becomes 0 getMaxKey() Return any key with the largest count getMinKey() Return any key with the smallest count If the data structure is empty, both...

Practice › LeetCode › LeetCode (archived) › LeetCode 03xx ›

LeetCode 381: Insert Delete GetRandom O(1) - Duplicates Allowed

Problem Restatement We need to implement a data structure called RandomizedCollection . It behaves like a multiset, so duplicate values are allowed. It must support three operations in average O(1) time: Operation Meaning insert(val) Insert one occurrence of val remove(val) Remove one occurrence of val if present getRandom() Return a random element from the current collection For getRandom() , every stored occurrence has equal probability. So if the collection contains:...

Practice › LeetCode › LeetCode (archived) › LeetCode 03xx ›

LeetCode 355: Design Twitter

Problem Restatement We need to design a simplified Twitter system. The system supports four operations: Method Meaning postTweet(userId, tweetId) User userId posts a tweet with ID tweetId getNewsFeed(userId) Return up to 10 most recent tweet IDs visible to userId follow(followerId, followeeId) followerId starts following followeeId unfollow(followerId, followeeId) followerId stops following followeeId A user's news feed contains: Tweets posted by the user themself. Tweets posted by users they follow. The feed...

Practice › LeetCode › LeetCode (archived) › LeetCode 03xx ›

LeetCode 380: Insert Delete GetRandom O(1)

Problem Restatement We need to implement a data structure called RandomizedSet . It supports three operations: Operation Meaning insert(val) Insert val if it does not already exist remove(val) Remove val if it exists getRandom() Return a random element from the current set Each operation must run in average O(1) time. For getRandom() , every element currently in the set must have the same probability of being returned. The problem guarantees...

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 155: Min Stack

Problem Restatement We need to design a stack that supports normal stack operations and also returns the minimum element currently inside the stack. The class must support these methods: Method Meaning MinStack() Initialize the stack push(val) Push val onto the stack pop() Remove the top element top() Return the top element getMin() Return the minimum element in the stack Each operation must run in O(1) time. LeetCode states that pop...

Practice › LeetCode › LeetCode (archived) › LeetCode 03xx ›

LeetCode 379: Design Phone Directory

Problem Restatement We need to design a phone directory. The directory starts with maxNumbers available numbers: 0, 1, 2, ..., maxNumbers - 1 We need to support three operations: Operation Meaning get() Return an available number and mark it as used check(number) Return whether number is available release(number) Make a used number available again If no number is available, get() must return -1 . The problem statement defines these operations...

Practice › LeetCode › LeetCode (archived) › LeetCode 03xx ›

LeetCode 303: Range Sum Query - Immutable

Problem Restatement We are given an integer array nums . We need to implement a class called NumArray that supports range sum queries. The class has two operations: Operation Meaning NumArray(nums) Build the object from the array sumRange(left, right) Return the sum of nums[left] + nums[left + 1] + ... + nums[right] The query range is inclusive, so both left and right are included. The array does not change after...

Practice › LeetCode › LeetCode (archived) › LeetCode 03xx ›

LeetCode 353: Design Snake Game

Problem Restatement We need to implement a simplified Snake game. The screen has size: height x width The snake starts at the top-left cell: (0, 0) Its initial length is 1 . We are given a list of food positions. Food appears one at a time. The second food appears only after the snake eats the first food. Each call to move(direction) moves the snake one cell in one of...

Practice › LeetCode › LeetCode (archived) › LeetCode 03xx ›

LeetCode 352: Data Stream as Disjoint Intervals

Problem Restatement We need to design a data structure called SummaryRanges . It receives non-negative integers from a data stream, one number at a time. After each insertion, it should be able to return all numbers seen so far as a sorted list of disjoint intervals. For example, if the stream gives: 1, 3, 7, 2, 6 Then the intervals change like this: after 1: [[1, 1]] after 3: [[1,...

Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 251: Flatten 2D Vector

Problem Restatement We need to design an iterator over a 2D vector. The input is a list of lists, for example: vec = [[1, 2], [3], [4]] The iterator should behave as if the numbers were stored in one flat list: [1, 2, 3, 4] The class must support two operations: Method Meaning next() Return the next integer and move the iterator forward hasNext() Return True if another integer exists,...

Programming › Algorithm Cookbook › Chapter 6. Sorting ›

6.25 Choosing the Right Sort

6.25 Choosing the Right Sort There is no single best sorting algorithm. The correct choice depends on input size, data characteristics, memory constraints, and required guarantees such as stability or worst-case bounds. Problem You need to select a sorting algorithm for a specific workload. The goal is to balance performance, memory usage, stability, and implementation complexity. Decision Factors Key dimensions that determine the choice: input size n data distribution key...

Programming › Algorithm Cookbook › Chapter 5. Hashing and Maps ›

5.24 Real-World Hash Table Design

5.24 Real-World Hash Table Design Problem You need to choose and implement a hash table that performs reliably under real workloads: mixed key types, uneven access patterns, deletions, resizing, and possible adversarial input. Pure theoretical designs often fail in production due to constant factors, memory layout, and edge cases. Solution Design the hash table as a layered system with explicit responsibilities: - hashing layer - indexing layer - collision handling...

Programming › Algorithm Cookbook › Chapter 5. Hashing and Maps ›

5.22 Hybrid Hash Structures

5.22 Hybrid Hash Structures Problem Pure hash tables do not handle all workloads well. They are fast on average, but can degrade under collisions, skewed key distributions, or heavy deletion patterns. You need a design that preserves hash-table speed while adapting to pathological cases. Solution Combine multiple internal representations depending on bucket state or load conditions. A common hybrid design: bucket: if small: use array or list if medium: use...

Programming › Algorithm Cookbook › Chapter 5. Hashing and Maps ›

5.2 Hash Functions

5.2 Hash Functions Problem You need a function that turns a key into a hash code so that a hash table can place the key into a bucket. A hash table depends on two operations. First, it must decide whether two keys are equal. Second, it must compute a hash code for each key. These two operations must agree. Whenever two keys are equal, they must produce the same hash...

#design

LeetCode 3885 - Design Event Manager

LeetCode 3829 - Design Ride Sharing System

LeetCode 3822 - Design Order Management System

LeetCode 3815 - Design Auction System

LeetCode 3709 - Design Exam Scores Tracker

LeetCode 3508 - Implement Router

LeetCode 3484 - Design Spreadsheet

LeetCode 3408 - Design Task Manager

LeetCode 2080 - Range Frequency Queries

LeetCode 3242 - Design Neighbor Sum Service

LeetCode 1865 - Finding Pairs With a Certain Sum

LeetCode 2069 - Walking Robot Simulation II

LeetCode 2166 - Design Bitset

LeetCode 2296 - Design a Text Editor

LeetCode 2349 - Design a Number Container System

LeetCode 2590 - Design a Todo List

LeetCode 2227 - Encrypt and Decrypt Strings

LeetCode 2102 - Sequentially Ordinal Rank Tracker

LeetCode 1912 - Design Movie Rental System

LeetCode 1804 - Implement Trie II (Prefix Tree)

LeetCode 2276 - Count Integers in Intervals

LeetCode 2408 - Design SQL

LeetCode 2353 - Design a Food Rating System

LeetCode 2502 - Design Memory Allocator

LeetCode 2671 - Frequency Tracker

LeetCode 2286 - Booking Concert Tickets in Groups

LeetCode 2526 - Find Consecutive Integers from a Data Stream

LeetCode 2254 - Design Video Sharing Platform

LeetCode 2241 - Design an ATM Machine

LeetCode 2043 - Simple Bank System

LeetCode 2642 - Design Graph With Shortest Path Calculator

LeetCode 3391 - Design a 3D Binary Matrix with Efficient Layer Tracking

LeetCode 1993 - Operations on Tree

LeetCode 2424 - Longest Uploaded Prefix

LeetCode 3369 - Design an Array Statistics Tracker

LeetCode 2336 - Smallest Number in Infinite Set

LeetCode 1348 - Tweet Counts Per Frequency

LeetCode 1476 - Subrectangle Queries

LeetCode 535 - Encode and Decode TinyURL

LeetCode 1586 - Binary Search Tree Iterator II

LeetCode 604 - Design Compressed String Iterator

LeetCode 1825 - Finding MK Average

LeetCode 588 - Design In-Memory File System

LeetCode 155 - Min Stack

LeetCode 716 - Max Stack

LeetCode 676 - Implement Magic Dictionary

LeetCode 1656 - Design an Ordered Stream

LeetCode 146 - LRU Cache

LeetCode 307 - Range Sum Query - Mutable

LeetCode 288 - Unique Word Abbreviation

LeetCode 1172 - Dinner Plate Stacks

LeetCode 1032 - Stream of Characters

LeetCode 705 - Design HashSet

LeetCode 1396 - Design Underground System

LeetCode 303 - Range Sum Query - Immutable

LeetCode 281 - Zigzag Iterator

LeetCode 384 - Shuffle an Array

LeetCode 211 - Design Add and Search Words Data Structure

LeetCode 677 - Map Sum Pairs

LeetCode 1157 - Online Majority Element In Subarray

LeetCode 173 - Binary Search Tree Iterator

LeetCode 1261 - Find Elements in a Contaminated Binary Tree

LeetCode 1357 - Apply Discount Every n Orders

LeetCode 901 - Online Stock Span

LeetCode 635 - Design Log Storage System

LeetCode 353 - Design Snake Game

LeetCode 1603 - Design Parking System

LeetCode 1244 - Design A Leaderboard

LeetCode 352 - Data Stream as Disjoint Intervals

LeetCode 284 - Peeking Iterator

LeetCode 933 - Number of Recent Calls

LeetCode 1600 - Throne Inheritance

LeetCode 855 - Exam Room

LeetCode 1483 - Kth Ancestor of a Tree Node

LeetCode 1472 - Design Browser History

LeetCode 622 - Design Circular Queue

LeetCode 703 - Kth Largest Element in a Stream

LeetCode 297 - Serialize and Deserialize Binary Tree

LeetCode 900 - RLE Iterator