CS foundations practice
Every CS foundations exercise, built from a topic you can open beside it.
164 exercises runs offline
All tracks Python 139 JavaScript 184 TypeScript 76 Go 106 Rust 75 Java 81 Kotlin 66 C++ 92 C# 88 Swift 68 PHP 69 Frontend 82 Backend 139 Architecture and system design 25 DevOps and cloud 15 Data and databases 5 Data science 31 AI and LLM engineering 75 Security 26 Game development 26 Coding in the AI era 101 CS foundations 164
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cs Quiz new Why can append to a geometrically growing dynamic array be amortized O(1) even though one… Estimate time and space growth with asymptotic notation and choose implementations that scale for real inputs. cs Quiz new A function performs 4n² + 20n + 300 modeled operations. What is its tight asymptotic time… Estimate time and space growth with asymptotic notation and choose implementations that scale for real inputs. cs Predict the output new What does this program print? Estimate time and space growth with asymptotic notation and choose implementations that scale for real inputs. cs Review AI code new Review generated dispatch batching Estimate time and space growth with asymptotic notation and choose implementations that scale for real inputs. cs Quiz new A linear scan stops when it finds its target. Which statement is precise? Estimate time and space growth with asymptotic notation and choose implementations that scale for real inputs. cs Spot the bug new Spot the bug in the duplicate check Estimate time and space growth with asymptotic notation and choose implementations that scale for real inputs. cs Quiz new What does amortized O(1) append mean for a geometrically growing dynamic array? Choose between contiguous arrays and linked nodes by access patterns, mutation costs, locality, and memory overhead. cs Quiz new When is insertion into a singly linked list truly O(1)? Choose between contiguous arrays and linked nodes by access patterns, mutation costs, locality, and memory overhead. cs Quiz new Why can an array traverse faster than a linked list even though both traversals are O(n)? Choose between contiguous arrays and linked nodes by access patterns, mutation costs, locality, and memory overhead. cs Predict the output new What does this access comparison print? Choose between contiguous arrays and linked nodes by access patterns, mutation costs, locality, and memory overhead. cs Review AI code new Review a generated recent-document list Choose between contiguous arrays and linked nodes by access patterns, mutation costs, locality, and memory overhead. cs Spot the bug new Spot the bug in the linked queue Choose between contiguous arrays and linked nodes by access patterns, mutation costs, locality, and memory overhead. cs Fill in new Name the recursive case that returns directly instead of making another recursive call Fill in cs Quiz new Which condition must hold before binary search can safely discard half of the remaining r… Quiz cs Fill in new Name the state a graph traversal must maintain to avoid revisiting vertices indefinitely … Fill in cs Quiz new A routine compares every pair among `n` records once. Which time bound describes its domi… Quiz cs Quiz new Which rule must a hash-map key implementation preserve? Quiz cs Fill in new Give the asymptotic time for indexed access to an array element Fill in cs Quiz new When is insertion into a linked list genuinely constant-time? Quiz cs Fill in new Name the traversal that finds a minimum-edge path in an unweighted graph Fill in cs Fill in new Name the failure mode in which ambiguous nested repetition makes a backtracking regular-e… Fill in cs Quiz new In JavaScript, which result does `0.1 + 0.2 === 0.3` produce? Quiz cs Quiz new Which validation rule prevents a regular expression from accepting a valid-looking substr… Quiz cs Quiz new Which localization approach preserves grammar across languages with different plural rule… Quiz cs Fill in new Name the JavaScript predicate that accepts only finite numbers without coercing its argum… Fill in cs Fill in new Name the Unicode normalization form that canonically decomposes and then recomposes text … Fill in cs Quiz new A UI limits a display name to 20 user-perceived characters. Which unit should it count? Quiz cs Quiz new Which design correctly handles elapsed deadlines and future local appointments? Quiz cs Fill in new Write the Bash expansion that forwards all positional parameters while preserving each ar… Fill in cs Quiz new A developer moved a local branch reference and can no longer see a recent commit in ordin… Quiz cs Quiz new Which statement correctly separates the Linux kernel from user space? Quiz cs Fill in new Name the lifecycle operation that waits for a worker thread or child process to finish an… Fill in cs Quiz new What does a page fault prove by itself? Quiz cs Quiz new What happens in `command >out.log 2>&1`? Quiz cs Quiz new Which statement describes the default memory relationship between threads in one process? Quiz cs Fill in new Give the conventional file-descriptor number for standard error Fill in cs Fill in new Name the transport unit whose message boundary UDP preserves Fill in cs Quiz new Which diagnostic statement about `NXDOMAIN` is correct? Quiz cs Fill in new Name the DNS record field that bounds how long a cache may ordinarily reuse the current a… Fill in cs Quiz new Which limitation remains after HTTP/2 multiplexes many streams over one TCP connection? Quiz cs Quiz new What does HTTP method idempotency mean for a retried request? Quiz cs Quiz new A sender makes two TCP `send()` calls. What may the receiver observe? Quiz cs Quiz new Which checks are required to authenticate the intended TLS service? Quiz cs Quiz new Which recovery design handles uncertainty after a WebSocket connection drops during a sid… Quiz cs Quiz new Which clock should control a five-second in-process timeout? Model instants, civil time, time zones, durations, and calendar arithmetic without hiding context in ambiguous conversions. cs Quiz new Which model best preserves a customer birthday entered as 2026-04-05? Model instants, civil time, time zones, durations, and calendar arithmetic without hiding context in ambiguous conversions. cs Quiz new Why can -04:00 not replace America/New_York in a recurring schedule? Model instants, civil time, time zones, durations, and calendar arithmetic without hiding context in ambiguous conversions. cs Predict the output new What does this program print for the two repeated New York times? Model instants, civil time, time zones, durations, and calendar arithmetic without hiding context in ambiguous conversions. cs Review AI code new Review a generated booking normalizer Model instants, civil time, time zones, durations, and calendar arithmetic without hiding context in ambiguous conversions. cs Spot the bug new Spot the bug in the time-zone conversion Model instants, civil time, time zones, durations, and calendar arithmetic without hiding context in ambiguous conversions. cs Quiz new Which sequence best supports a DNS address migration with a previous TTL of one hour? Trace names through recursive and authoritative DNS, caching, record types, TTLs, and failure modes. cs Quiz new A `NOERROR` response contains authority evidence but no `AAAA` record for an existing nam… Trace names through recursive and authoritative DNS, caching, record types, TTLs, and failure modes. cs Predict the output new What does this cache-boundary check print? Trace names through recursive and authoritative DNS, caching, record types, TTLs, and failure modes. cs Quiz new Which statement correctly separates recursive and authoritative DNS roles? Trace names through recursive and authoritative DNS, caching, record types, TTLs, and failure modes. cs Review AI code new Review a generated URL resolver Trace names through recursive and authoritative DNS, caching, record types, TTLs, and failure modes. cs Spot the bug new Spot the bug in the DNS endpoint lookup Trace names through recursive and authoritative DNS, caching, record types, TTLs, and failure modes. cs Quiz new Why can (a + b) + c differ from a + (b + c) for finite floating-point values? Predict representation error, rounding, overflow, and reliable comparisons in binary floating-point calculations. cs Quiz new What does Number.EPSILON describe? Predict representation error, rounding, overflow, and reliable comparisons in binary floating-point calculations. cs Quiz new An API accepts only ordinary finite measurements. Which boundary check states that policy… Predict representation error, rounding, overflow, and reliable comparisons in binary floating-point calculations. cs Predict the output new What does this program print? Predict representation error, rounding, overflow, and reliable comparisons in binary floating-point calculations. cs Review AI code new Review generated sensor comparison Predict representation error, rounding, overflow, and reliable comparisons in binary floating-point calculations. cs Spot the bug new Spot the bug in measurement comparison Predict representation error, rounding, overflow, and reliable comparisons in binary floating-point calculations. cs Quiz new A bad commit is already on a protected branch used by teammates. Which default recovery p… Understand Git through objects, references, the index, and the commit graph, then rebase, undo, and recover work safely. cs Predict the output new After committing 10, staging 20, and editing 30, what does this script print? Understand Git through objects, references, the index, and the commit graph, then rebase, undo, and recover work safely. cs Review AI code new Review generated release script Understand Git through objects, references, the index, and the commit graph, then rebase, undo, and recover work safely. cs Spot the bug new Spot the bug in the Git sync script Understand Git through objects, references, the index, and the commit graph, then rebase, undo, and recover work safely. cs Quiz new Which Git object associates `src/app.js` with its content object and file mode? Understand Git through objects, references, the index, and the commit graph, then rebase, undo, and recover work safely. cs Quiz new Two unequal keys select the same bucket. What must a correct hash map do? Use hashing, collision handling, resizing, and key equality to build and evaluate constant-time lookup tables. cs Quiz new Which statement is required by a valid hash-equality contract? Use hashing, collision handling, resizing, and key equality to build and evaluate constant-time lookup tables. cs Predict the output new What does this JavaScript program print? Use hashing, collision handling, resizing, and key equality to build and evaluate constant-time lookup tables. cs Quiz new Why must entries normally be relocated when a hash table changes capacity? Use hashing, collision handling, resizing, and key equality to build and evaluate constant-time lookup tables. cs Review AI code new Review an untrusted request index Use hashing, collision handling, resizing, and key equality to build and evaluate constant-time lookup tables. cs Spot the bug new Spot the bug in the string map Use hashing, collision handling, resizing, and key equality to build and evaluate constant-time lookup tables. cs Quiz new A cache sends `If-None-Match` for a stored GET response and receives `304 Not Modified`. … Design and diagnose HTTP exchanges using methods, status codes, fields, caching, idempotency, and connection boundaries. cs Quiz new What does `Cache-Control: no-cache` require for a stored response? Design and diagnose HTTP exchanges using methods, status codes, fields, caching, idempotency, and connection boundaries. cs Predict the output new What does this simplified PUT contract print? Design and diagnose HTTP exchanges using methods, status codes, fields, caching, idempotency, and connection boundaries. cs Quiz new A connection resets after a client finishes sending a write but before it reads a respons… Design and diagnose HTTP exchanges using methods, status codes, fields, caching, idempotency, and connection boundaries. cs Review AI code new Review a generated conditional profile handler Design and diagnose HTTP exchanges using methods, status codes, fields, caching, idempotency, and connection boundaries. cs Spot the bug new Spot the bug in the payment retry Design and diagnose HTTP exchanges using methods, status codes, fields, caching, idempotency, and connection boundaries. cs Quiz new A sender has stream-window credit but no connection-window credit. May it send another DA… HTTP/2 carries concurrent requests over one connection with binary frames, multiplexing, and HPACK; learn its streams, flow control, and failure boundaries. cs Quiz new After receiving GOAWAY, what is the safest general retry rule? HTTP/2 carries concurrent requests over one connection with binary frames, multiplexing, and HPACK; learn its streams, flow control, and failure boundaries. cs Predict the output new What does this frame-header inspection print? HTTP/2 carries concurrent requests over one connection with binary frames, multiplexing, and HPACK; learn its streams, flow control, and failure boundaries. cs Review AI code new Review generated HTTP/2 batch client HTTP/2 carries concurrent requests over one connection with binary frames, multiplexing, and HPACK; learn its streams, flow control, and failure boundaries. cs Spot the bug new Spot the bug in the HTTP/2 response helper HTTP/2 carries concurrent requests over one connection with binary frames, multiplexing, and HPACK; learn its streams, flow control, and failure boundaries. cs Quiz new What is the soundest way to adapt an HTML application to an RTL locale? Internationalization makes locale, time zone, plural rules, and writing direction explicit inputs so one product can serve different regions correctly. cs Predict the output new What does this program print? Internationalization makes locale, time zone, plural rules, and writing direction explicit inputs so one product can serve different regions correctly. cs Review AI code new Review generated invoice renderer Internationalization makes locale, time zone, plural rules, and writing direction explicit inputs so one product can serve different regions correctly. cs Quiz new Which inputs correctly describe formatting an order total? Internationalization makes locale, time zone, plural rules, and writing direction explicit inputs so one product can serve different regions correctly. cs Spot the bug new Spot the bug in the localized welcome Internationalization makes locale, time zone, plural rules, and writing direction explicit inputs so one product can serve different regions correctly. cs Quiz new Which permission most directly controls removing a file name from a directory? Understand Linux kernel boundaries, files and permissions, processes, descriptors, and signals, then troubleshoot them with verifiable commands. cs Quiz new Why should a service controller normally send SIGTERM before SIGKILL? Understand Linux kernel boundaries, files and permissions, processes, descriptors, and signals, then troubleshoot them with verifiable commands. cs Predict the output new What does this permission script print? Understand Linux kernel boundaries, files and permissions, processes, descriptors, and signals, then troubleshoot them with verifiable commands. cs Review AI code new Review generated deployment script Understand Linux kernel boundaries, files and permissions, processes, descriptors, and signals, then troubleshoot them with verifiable commands. cs Spot the bug new Spot the bug in the cleanup function Understand Linux kernel boundaries, files and permissions, processes, descriptors, and signals, then troubleshoot them with verifiable commands. cs Predict the output new What does this local stream-socket program print? Start with socket, TCP, and UDP semantics, then handle framing, timeouts, shutdown, and backpressure without ambiguous boundaries. cs Review AI code new Review generated JSON command server Start with socket, TCP, and UDP semantics, then handle framing, timeouts, shutdown, and backpressure without ambiguous boundaries. cs Spot the bug new Spot the bug in the framed message reader Start with socket, TCP, and UDP semantics, then handle framing, timeouts, shutdown, and backpressure without ambiguous boundaries. cs Quiz new A TCP sender calls sendall() twice with two application messages. What may the receiver o… Start with socket, TCP, and UDP semantics, then handle framing, timeouts, shutdown, and backpressure without ambiguous boundaries. cs Quiz new What happens when a UDP datagram is larger than the buffer passed to a basic recvfrom() c… Start with socket, TCP, and UDP semantics, then handle framing, timeouts, shutdown, and backpressure without ambiguous boundaries. cs Quiz new Which statement about `value=$(command)` in Bash is correct? Compose commands with standard streams, pipelines, redirection, and exit handling without losing errors or data. cs Quiz new When does a blocking pipe reader observe end-of-file? Compose commands with standard streams, pipelines, redirection, and exit handling without losing errors or data. cs Quiz new In Bash, `producer | consumer` runs with `pipefail` disabled. The producer exits 7 and th… Compose commands with standard streams, pipelines, redirection, and exit handling without losing errors or data. cs Predict the output new What does this Bash script print? Compose commands with standard streams, pipelines, redirection, and exit handling without losing errors or data. cs Review AI code new Review a generated export pipeline Compose commands with standard streams, pipelines, redirection, and exit handling without losing errors or data. cs Spot the bug new Spot the bug in the export pipeline Compose commands with standard streams, pipelines, redirection, and exit handling without losing errors or data. cs Quiz new A thread blocked on socket input receives data. What is the most accurate next-state clai… Distinguish processes and threads, trace their lifecycle and shared state, and reason about scheduling and synchronization. cs Quiz new What does joining a thread guarantee that requesting cancellation alone does not? Distinguish processes and threads, trace their lifecycle and shared state, and reason about scheduling and synchronization. cs Predict the output new What does this shared-memory code print? Distinguish processes and threads, trace their lifecycle and shared state, and reason about scheduling and synchronization. cs Quiz new Which statement most accurately distinguishes a process from a thread? Distinguish processes and threads, trace their lifecycle and shared state, and reason about scheduling and synchronization. cs Review AI code new Review generated report workers Distinguish processes and threads, trace their lifecycle and shared state, and reason about scheduling and synchronization. cs Spot the bug new Spot the bug in the shared counter update Distinguish processes and threads, trace their lifecycle and shared state, and reason about scheduling and synchronization. cs Quiz new A recursive depth-first traversal visits every node in a balanced binary tree with n node… Define recursive cases, prove termination, trace the call stack, and replace recursion when depth is unsafe. cs Quiz new A recursive pre-order traversal visits children from left to right. How should an equival… Define recursive cases, prove termination, trace the call stack, and replace recursion when depth is unsafe. cs Predict the output new What does this program print? Define recursive cases, prove termination, trace the call stack, and replace recursion when depth is unsafe. cs Review AI code new Review a generated category indexer Define recursive cases, prove termination, trace the call stack, and replace recursion when depth is unsafe. cs Spot the bug new Spot the bug in the graph search Define recursive cases, prove termination, trace the call stack, and replace recursion when depth is unsafe. cs Quiz new Which argument proves that recursive binary search on [low, high) terminates? Define recursive cases, prove termination, trace the call stack, and replace recursion when depth is unsafe. cs Quiz new Which approach explicitly requires an order-reference match to consume the entire JavaScr… Build, read, test, and constrain regular expressions for reliable validation, search, capture, and replacement while avoiding escape and performance traps. cs Predict the output new What does this global search print? Build, read, test, and constrain regular expressions for reliable validation, search, capture, and replacement while avoiding escape and performance traps. cs Quiz new Why is a replacement callback safer when untrusted replacement text may contain dollar si… Build, read, test, and constrain regular expressions for reliable validation, search, capture, and replacement while avoiding escape and performance traps. cs Review AI code new Review a generated ticket scanner Build, read, test, and constrain regular expressions for reliable validation, search, capture, and replacement while avoiding escape and performance traps. cs Spot the bug new Spot the bug in the global email validator Build, read, test, and constrain regular expressions for reliable validation, search, capture, and replacement while avoiding escape and performance traps. cs Quiz new A field permits decimal digits from every Unicode script. Which ECMAScript atom expresses… Build, read, test, and constrain regular expressions for reliable validation, search, capture, and replacement while avoiding escape and performance traps. cs Quiz new When should a script save the special parameter whose name is `$?`? Navigate files, quote arguments, expand variables, inspect exit status, and run commands safely in a shell. cs Quiz new A script exports REGION=eu and starts a child process. The child changes REGION to us. Wh… Navigate files, quote arguments, expand variables, inspect exit status, and run commands safely in a shell. cs Quiz new Which expansion forwards all positional parameters while preserving each original argumen… Navigate files, quote arguments, expand variables, inspect exit status, and run commands safely in a shell. cs Predict the output new What does this Bash program print? Navigate files, quote arguments, expand variables, inspect exit status, and run commands safely in a shell. cs Review AI code new Review a generated report archiver Navigate files, quote arguments, expand variables, inspect exit status, and run commands safely in a shell. cs Spot the bug new Spot the bug in the copy helper Navigate files, quote arguments, expand variables, inspect exit status, and run commands safely in a shell. cs Quiz new You need the first matching record from a small unsorted array, and this query runs once.… Choose and implement sorting and searching strategies from data shape, ordering guarantees, and complexity. cs Predict the output new What does this program print? Choose and implement sorting and searching strategies from data shape, ordering guarantees, and complexity. cs Review AI code new Review a generated price search Choose and implement sorting and searching strategies from data shape, ordering guarantees, and complexity. cs Quiz new Binary search finds an insertion position in a sorted array in O(log n). Why can the comp… Choose and implement sorting and searching strategies from data shape, ordering guarantees, and complexity. cs Spot the bug new Spot the bug in response-time ranking Choose and implement sorting and searching strategies from data shape, ordering guarantees, and complexity. cs Quiz new What does a stable sort guarantee when two distinct records compare equal? Choose and implement sorting and searching strategies from data shape, ordering guarantees, and complexity. cs Quiz new A certificate builds successfully to a trusted root but names only payments.example. What… Verify how certificates, hostnames, trust chains, handshakes, and key agreement protect connections. cs Quiz new Which operation is the best candidate for TLS 1.3 0-RTT early data? Verify how certificates, hostnames, trust chains, handshakes, and key agreement protect connections. cs Quiz new What does ephemeral Diffie-Hellman key agreement establish by itself? Verify how certificates, hostnames, trust chains, handshakes, and key agreement protect connections. cs Predict the output new What does this certificate-name check print? Verify how certificates, hostnames, trust chains, handshakes, and key agreement protect connections. cs Review AI code new Review a generated invoice TLS client Verify how certificates, hostnames, trust chains, handshakes, and key agreement protect connections. cs Spot the bug new Spot the bug in the TLS client Verify how certificates, hostnames, trust chains, handshakes, and key agreement protect connections. cs Quiz new Why does an adjacency list usually fit a sparse graph better than an adjacency matrix? Model hierarchical and connected data, then traverse it with depth-first and breadth-first strategies. cs Quiz new When does ordinary BFS provide a shortest-path guarantee? Model hierarchical and connected data, then traverse it with depth-first and breadth-first strategies. cs Predict the output new What does this iterative DFS print? Model hierarchical and connected data, then traverse it with depth-first and breadth-first strategies. cs Review AI code new Review a generated dependency order Model hierarchical and connected data, then traverse it with depth-first and breadth-first strategies. cs Quiz new Which property distinguishes a rooted tree from a general directed graph? Model hierarchical and connected data, then traverse it with depth-first and breadth-first strategies. cs Spot the bug new Spot the bug in the reachability helper Model hierarchical and connected data, then traverse it with depth-first and breadth-first strategies. cs Quiz new A protocol allows at most 64 UTF-8 bytes for a display label. Which check enforces that c… Handle code points, encodings, grapheme clusters, normalization, and text comparison without corrupting user data. cs Quiz new A French Intl.Collator with sensitivity: "base" returns 0 for two account names. What can… Handle code points, encodings, grapheme clusters, normalization, and text comparison without corrupting user data. cs Quiz new Why should a program not replace NFC with NFKC everywhere? Handle code points, encodings, grapheme clusters, normalization, and text comparison without corrupting user data. cs Predict the output new What does this program print? Handle code points, encodings, grapheme clusters, normalization, and text comparison without corrupting user data. cs Review AI code new Review a generated customer importer Handle code points, encodings, grapheme clusters, normalization, and text comparison without corrupting user data. cs Spot the bug new Spot the bug in the text preview Handle code points, encodings, grapheme clusters, normalization, and text comparison without corrupting user data. cs Quiz new After fork, why can a child write a private mapping without changing the parent’s value? Reason about address spaces, pages, allocation, caching, swapping, and memory faults in running programs. cs Quiz new Which statement about a page fault is correct? Reason about address spaces, pages, allocation, caching, swapping, and memory faults in running programs. cs Predict the output new What does this page-stride loop print? Reason about address spaces, pages, allocation, caching, swapping, and memory faults in running programs. cs Review AI code new Review a generated mapped index Reason about address spaces, pages, allocation, caching, swapping, and memory faults in running programs. cs Spot the bug new Spot the bug in the memory mapping helper Reason about address spaces, pages, allocation, caching, swapping, and memory faults in running programs. cs Quiz new A process reserves a 2 GiB anonymous range but touches only 8 MiB. Which conclusion is ju… Reason about address spaces, pages, allocation, caching, swapping, and memory faults in running programs. cs Quiz new A browser application must verify that its server-side business loop is responsive. Which… WebSocket carries two-way messages on one connection; learn its handshake, boundaries, closing, recovery, security, and backpressure. cs Predict the output new Which two booleans does this send-admission check print? WebSocket carries two-way messages on one connection; learn its handshake, boundaries, closing, recovery, security, and backpressure. cs Quiz new A client loses its connection after sending a payment command but before receiving an ack… WebSocket carries two-way messages on one connection; learn its handshake, boundaries, closing, recovery, security, and backpressure. cs Review AI code new Review generated browser chat client WebSocket carries two-way messages on one connection; learn its handshake, boundaries, closing, recovery, security, and backpressure. cs Spot the bug new Spot the bug in the send helper WebSocket carries two-way messages on one connection; learn its handshake, boundaries, closing, recovery, security, and backpressure.
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