CS fundamentals and algorithms interview questions

CS fundamentals and algorithmsShort answerIntroAppleinterview report

Stacks and queues

What's the difference between a stack and a queue?

#stack #queue #data-structure #navigation

CS fundamentals and algorithmsMultiple choiceIntroSnapinterview report

Array vs linked list

You're picking a data structure for the message list on a chat screen. Which is the most accurate reason a Swift Array beats a linked list here?

#array #linked-list #complexity

Swift collection complexity

Which of these Swift collection operations have O(1) average time complexity? Select all that apply.

#complexity #array #set #dictionary

CS fundamentals and algorithmsShort answerIntro카카오

TCP vs UDP

What's the difference between TCP and UDP? When a packet is lost, what happens under each one?

#tcp #udp #network #urlsession

Image formats

Which statements about image formats are correct? Select all that apply.

#png #jpeg #heic #webp

CS fundamentals and algorithmsMultiple choiceIntroZomato

Interval overlap

You're writing a function that decides whether two text highlight ranges overlap. For two half-open ranges a = [a.lower, a.upper) and b = [b.lower, b.upper), w…

#range #interval #overlap #nsrange

CS fundamentals and algorithmsShort answerIntroZoom

Linked list intersection

Two singly linked lists may share their tail nodes. How do you find the first node they actually share (the same instance, not a node with an equal value) in O…

#linked-list #two-pointers #identity #algorithm

CS fundamentals and algorithmsShort answerIntroZalandointerview report

Balanced brackets

A Swift string mixes ()[]{} and you're checking with a stack whether they nest correctly. When you hit a closing bracket, what comparison do you make?

#stack #brackets #string #algorithm

Array insert cost

A live feed calls posts.insert(newPost, at: 0) every time a new post arrives. What goes wrong once 50,000 posts have piled up?

#array #complexity #prepend #deque

CS fundamentals and algorithmsShort answerBasicsMetainterview report

Hash maps and Dictionary

If you had to implement a hash map yourself, how would you build it?

#hashmap #dictionary #hashable #collision

Hashable consistency

What problem does this type cause when you put it in a Set? …

#hashable #equatable #set #bug

Bit operations and OptionSet

Given this code, which statements about OptionSet and bit operations are correct? Select all that apply. …

#bitwise #optionset #bitmask #enum

Choosing a search algorithm

You're building name search over a list of 10,000 contacts. How do linear search and sort-then-binary-search compare, and what would you actually use in a ship…

#binary-search #linear-search #dictionary #trie

Processes and threads

What's the difference between a process and a thread? What do threads share, and what does each one keep to itself?

#process #thread #os #extension

Thread explosion

Running this code, which downloads 200 URLs with GCD, sends the system thread count past 60 and freezes the app. What's the most accurate cause? …

#thread-explosion #gcd #concurrent-queue #semaphore

CS fundamentals and algorithmsShort answerBasicsSnapinterview report

Sorting and Swift sort

Can you compare merge sort and quicksort: how each works, and their time and space complexity?

#sort #stable-sort #merge-sort #quick-sort

Stable sorting

You take a contacts array already sorted by name and re-sort it with contacts.sort { $0.isFavorite && !$1.isFavorite }, and the names inside the favorites grou…

#sort #stable-sort #ui

Pure functions

Which of these are pure functions (same output for the same input, no side effects)? Select all that apply.

#pure-function #functional #side-effect #swiftui

CS fundamentals and algorithmsShort answerBasicsApple

O(1) max stack

You want a stack, like an editor's undo stack, that returns the "current maximum" of the pushed values in O(1). First define the interface as a Swift protocol,…

#stack #max-stack #protocol #xctest

CS fundamentals and algorithmsShort answerBasicsMeta

Streaming moving average

Frame times arrive as a stream, one per frame. Design a type that returns "the average of the last k frames" instantly whenever asked. It's called 60 times a s…

#ring-buffer #sliding-window #moving-average #fps

CS fundamentals and algorithmsShort answerBasicsReddit

Anchored two-way lookup

Chat messages sit in an array sorted by id ascending. When the user taps a push notification to jump to a specific message, how would you write a function that…

#binary-search #sorted-array #pagination #anchor

CS fundamentals and algorithmsMultiple choiceBasicsPayPal

Sliding window preconditions

You solved "longest contiguous run of transaction amounts whose sum stays at or under a limit L" in O(n) with a two-pointer sliding window. The interviewer ask…

#sliding-window #two-pointers #prefix-sum #monotonic

CS fundamentals and algorithmsShort answerBasicsAudible

Rearranging without adjacent duplicates

You have a list of posts for a feed and need to reorder them so that no two posts by the same author appear back to back. What algorithm do you use, and what's…

#greedy #heap #priority-queue #feed

CS fundamentals and algorithmsShort answerBasicsMastercard

Parsing corrupted logs

Your app writes a network log file. Each line has the format userID(4 digits) pageName startTime(hh:mm:ss) endTime(hh:mm:ss), but some lines are corrupted. How…

#parsing #validation #log #top-k

CS fundamentals and algorithmsMultiple choiceBasicsJPMorgan Chase

Time window anomaly detection

A list of transactions (card number, country, time) arrives sorted by time ascending. What's the most efficient way to detect "the same card used in two differ…

#hashmap #time-window #stream #fraud

CS fundamentals and algorithmsMultiple choiceBasicsCapital Oneinterview report

Luhn checksum

On a card number entry screen you want to validate the number with the Luhn algorithm before sending it to the server. Which statements about Luhn validation a…

#luhn #checksum #validation #card-number

CS fundamentals and algorithmsShort answerBasicsPinterestinterview report

Infinite scroll cursors

In an endless iOS list like a pin board, you want to stop the same item showing twice or the next page going missing. If you use a cursor instead of an offset,…

#pagination #cursor #infinite-scroll #deduplication

CS fundamentals and algorithmsShort answerBasicsSquare

Inventory domain design

You're designing a Swift inventory system that processes receive, reserve, sell, and cancel operations in sequence. What domain type protects the invariants on…

#object-oriented-design #inventory #invariant #domain-model

CS fundamentals and algorithmsShort answerBasicsBilibili

Base-36 string addition

You need to add two base-36 strings made of digits and letters without converting them to integers (so no overflow) and return the result as a string. How do y…

#string #base36 #carry #arithmetic

CS fundamentals and algorithmsShort answerBasicsDeloitteinterview report

O(1) LRU cache

You're designing an LRU cache that keeps only recently used items. To make get and put average O(1), what does the HashMap do and what does the doubly linked l…

#lru-cache #hash-map #linked-list #data-structure

CS fundamentals and algorithmsShort answerBasics오늘의집

Shortest path with BFS

A home decor recommender follows a graph of room-to-room connections to find the minimum number of moves. When every edge costs the same, why does BFS guarante…

#graph #bfs #visited-set #shortest-path

CS fundamentals and algorithmsShort answerBasicsN26interview report

HashMap vs Set

In a one-hour coding test you have to find duplicates in a list of customer IDs and also count how often each ID appears. When would you use Swift's Set and wh…

#hashmap #set #complexity #swift

LRU cache

You need to write an in-memory image cache with LRU eviction yourself. Which data structures give you O(1) get and put?

#lru #cache #linked-list #dictionary

Floating point and Decimal

In payment amount math, 0.1 + 0.2 == 0.3 came back false and a test failed. Which statements about floating point and Decimal are correct? Select all that appl…

#floating-point #decimal #ieee754 #currency

QUIC and mobile networks

A user on the subway keeps bouncing between Wi-Fi and LTE, and your app's requests keep dropping. How do HTTP/2 over TCP and HTTP/3 over QUIC each handle that?

#quic #http3 #tcp #connection-migration

First request latency

On a cold start, your app's first API response takes 800ms and every one after that takes 120ms. Can you walk me through what happens in the network stack in b…

#dns #tls #handshake #latency

CS fundamentals and algorithmsShort answerAdvancedMetainterview report

View hierarchy tree search

You're given a specific UIView instance in an original view hierarchy, plus a copy of that hierarchy with the same structure. How would you design a function t…

#bfs #dfs #tree #view-hierarchy

CS fundamentals and algorithmsShort answerAdvancedTikTok

Recursion and stack limits

You're recursively walking a nested JSON document thousands of levels deep, and it crashes only on a background thread. What's the cause?

#recursion #iteration #stack-overflow #thread-stack

Cache locality

You iterate over 100,000 coordinates and sum distances. Between these two versions, what's the main reason A is noticeably faster than B? …

#cache-locality #struct #class #memory-layout

CS fundamentals and algorithmsShort answerAdvancedGoogle

K nearest places

You have tens of thousands of places with an id and coordinates (latitude, longitude). Given the user's current location, design a function that finds the 10 n…

#k-nearest #heap #quickselect #geo

CS fundamentals and algorithmsShort answerAdvancedPayPal

LRU cache with TTL

You've implemented an LRU cache and the interviewer follows up: "Now make each entry expire a fixed time after it's stored. What gets more expensive?" Explain …

#lru #ttl #cache #expiry

CS fundamentals and algorithmsShort answerAdvancedMicrosoft

Connected components, recursion limits

An image mask comes as an N×M grid where 1 is foreground and 0 is background. How would you write a function that returns an array with the size of each blob o…

#flood-fill #bfs #dfs #matrix

CS fundamentals and algorithmsShort answerAdvancedShopee

Purpose of virtual memory

Someone asks you, "If a device has plenty of physical memory, why do we even need virtual memory?" How would you answer?

#virtual-memory #paging #mmap #jetsam

CS fundamentals and algorithmsShort answerAdvancedAlibaba Group

Deduplicating beyond memory

You have a 1TB log file with one URL per line and need to remove duplicate URLs, but you only have 1GB of memory. How do you do it?

#external-memory #hash-partition #bloom-filter #dedupe

Diff algorithms

When you apply a new snapshot to UICollectionViewDiffableDataSource, how does it compute the inserts, deletes, and moves? And why do item identifiers have to b…

#diffable-data-source #diff #myers #heckel

Topological sort of dependencies

In an app split into 30 modules, the DI container initializes services in an order someone maintains by hand, and it crashed with "A needs B and B needs A." Wh…

#graph #topological-sort #dependency #cycle-detection

Trie based autocomplete

You need search autocomplete that works offline: 500,000 candidate words, and on every keystroke you return the top 10 within 16ms. How do you design the data …

#trie #autocomplete #memory #search

Prefix sum binary search

You're writing a custom vertical layout (a UICollectionViewLayout subclass) where every cell has a different height. layoutAttributesForElements(in rect:) gets…

#prefix-sum #binary-search #layout #scroll