Software Engineer Interview Flashcards

Question 1

What is the average time complexity of looking up a value by key in a hash table?

Accepted Answer

O(1) on average. The key is hashed to a bucket index, so a lookup goes straight to the slot instead of scanning. Worst case is O(n) when many keys collide into one bucket, which good hash functions and resizing keep rare.

Question 2

What is the time complexity of binary search on a sorted array?

Accepted Answer

O(log n) time and O(1) extra space when done iteratively. Each comparison halves the remaining search space, so it takes about log2(n) steps. It requires the array to be sorted first.

Question 3

Which data structure follows Last-In-First-Out (LIFO) order?

Accepted Answer

A stack: the most recently pushed element is the first one popped (LIFO). Push and pop are O(1). Common uses include call stacks, undo history, and depth-first traversal.

Question 4

Which order does a queue follow?

Accepted Answer

FIFO: the earliest enqueued element is dequeued first. Enqueue and dequeue are O(1). Used for task scheduling, buffering, and breadth-first traversal.

Question 5

Which gives O(1) random access by index: an array or a linked list?

Accepted Answer

An array gives O(1) random access because elements are stored contiguously, so address = base + index x element_size. A linked list needs O(n) traversal to reach an index since nodes are scattered and connected by pointers.

Question 6

In JavaScript, what is the difference between null and undefined?

Accepted Answer

undefined is the default for a declared-but-unassigned variable, a missing property, or a function with no return. null is an explicit 'no value' you assign yourself. Note typeof undefined is 'undefined' but typeof null is 'object', and null == undefined is true while null === undefined is false.

Question 7

What is the main semantic difference between HTTP GET and POST?

Accepted Answer

GET is safe and idempotent: it only retrieves data, carries parameters in the URL, and is cacheable. POST submits data in the request body to create or change server state, is not idempotent, and is not cached by default.

Question 8

What does it mean for an HTTP method to be idempotent?

Accepted Answer

An idempotent method leaves the server in the same state no matter how many times the identical request is repeated. GET, PUT, and DELETE are idempotent; POST generally is not. This is what makes safe retries after network failures possible.

Question 9

What does HTTP status code 404 mean?

Accepted Answer

404 Not Found: a client error (4xx) meaning the server was reached but could not find the requested resource. It differs from 410 Gone (intentionally removed) and 401/403 (authentication or authorization failures).

Question 10

What does HTTP status code 500 mean?

Accepted Answer

500 Internal Server Error: a generic server-side failure (5xx) where an unexpected condition, such as an unhandled exception, prevented fulfilling the request. Unlike 4xx client errors, the fault is on the server.

Question 11

What is the difference between a compiled and an interpreted language?

Accepted Answer

A compiled language (C, Rust) is translated ahead of time into machine code, giving fast execution but a build step. An interpreted language (Python) executes statement-by-statement at runtime, trading speed for flexibility. Many modern languages blend both with bytecode and JIT compilation.

Question 12

What is the difference between git commit and git push?

Accepted Answer

git commit records a snapshot of staged changes in your local repository history. git push uploads those local commits to a remote (e.g. origin) so others can see them. You can commit many times offline, then push them together.

Question 13

What causes a git merge conflict?

Accepted Answer

A conflict arises when two branches make incompatible changes to the same lines (or one edits a file the other deleted), so Git cannot merge automatically. It marks the regions with <<<<<<<, =======, and >>>>>>> for you to resolve manually, then commit.

Question 14

What is a primary key in a relational database?

Accepted Answer

A primary key uniquely identifies each row in a table: it must be unique and non-null, and there is exactly one per table. It is typically backed by an index and is the target of foreign keys in other tables.

Question 15

What is a foreign key?

Accepted Answer

A foreign key is a column (or set) referencing another table's primary key, enforcing referential integrity: you cannot insert a value that does not exist in the parent table, and deletes can cascade or be restricted. It models relationships between tables.

Question 16

What does a SQL INNER JOIN return?

Accepted Answer

An INNER JOIN returns only rows where the join condition matches in both tables; non-matching rows are dropped. Contrast with a LEFT JOIN, which keeps all left-table rows and fills NULLs where the right side has no match.

Question 17

Is an empty string truthy or falsy in most languages like JavaScript and Python?

Accepted Answer

Falsy: an empty string evaluates to false in a boolean context in both JavaScript and Python. Other falsy values include 0, null/None, undefined, NaN, and (in Python) empty collections.

Question 18

What is the difference between local and global scope?

Accepted Answer

A local variable lives only within the function or block where it is declared and is destroyed when that scope exits. A global variable is reachable throughout the program. Prefer local scope to avoid naming collisions and unintended side effects.

Question 19

Why does a recursive function need a base case?

Accepted Answer

The base case is the condition that returns without another recursive call, stopping the recursion. Without it, or if inputs never reach it, the function calls itself indefinitely and grows the call stack until a stack overflow. Each step must move toward the base case.

Question 20

What does it mean for an object to be immutable?

Accepted Answer

An immutable object cannot be modified after creation; any 'change' returns a new object (e.g. Python strings and tuples, JavaScript strings). Immutability makes code easier to reason about, safe to share across threads, and usable as hash keys.

Question 21

What is the purpose of a try/catch block?

Accepted Answer

try/catch runs code that may throw and handles the exception gracefully rather than crashing. The try block holds the risky code, catch handles the error, and an optional finally block runs cleanup regardless of the outcome.

Question 22

What does REST stand for?

Accepted Answer

REST = Representational State Transfer, an architectural style for networked APIs. Its key ideas are stateless requests, resources identified by URLs, standard HTTP verbs (GET/POST/PUT/DELETE), and representations such as JSON.

Question 23

What is JSON used for?

Accepted Answer

JSON (JavaScript Object Notation) is a lightweight, language-agnostic text format for structured data using key-value pairs and arrays. It is the common choice for API payloads and config because it is human-readable and maps cleanly to most languages' data types.

Question 24

What does the logical AND operator return when one operand is false?

Accepted Answer

false, because AND requires both operands to be true, so a single false forces the result false. With short-circuit evaluation, the operator stops at the first false and skips the rest.

Question 25

What is the difference between a compile-time and a runtime error?

Accepted Answer

A compile-time error such as a syntax or type error is caught by the compiler before the program runs. A runtime error such as a divide-by-zero or null dereference occurs during execution with otherwise valid syntax. Runtime errors are harder to find because they depend on inputs and state.

Question 26

What does concatenation do to two strings?

Accepted Answer

Concatenation joins two strings end-to-end into one new string ('foo' + 'bar' becomes 'foobar'). Because strings are usually immutable, it allocates a new string, so building many in a loop is better done with a builder or join for performance.

Question 27

What is a constant in programming?

Accepted Answer

A constant is a named value that cannot be reassigned after initialization (const in JavaScript, final in Java). It signals intent and prevents accidental changes. Note that in some languages const only blocks reassignment, not mutation of the referenced object.

Question 28

Name one advantage of an iterative loop over recursion.

Accepted Answer

Iteration typically uses constant stack space, while recursion adds a stack frame per call and risks stack overflow on deep inputs. Loops also avoid function-call overhead. Recursion can still be clearer for inherently recursive problems like tree traversal.

Question 29

What is the difference between a parameter and an argument?

Accepted Answer

A parameter is the named placeholder in a function's definition; an argument is the actual value supplied at the call site. In def f(x), x is a parameter; in f(5), 5 is the argument bound to x.

Question 30

What is the purpose of code comments?

Accepted Answer

Comments explain intent, rationale, or non-obvious logic to human readers and are ignored by the compiler or interpreter. Good comments explain why rather than restating what the code already shows, and outdated comments are worse than none, so keep them in sync.

Question 31

What does the NOT operator do to a boolean value?

Accepted Answer

NOT (!) inverts a boolean: !true is false and !false is true. Applied to non-booleans it first coerces to a boolean (so NOT of an empty string is true). Double negation (!!x) is a common idiom for coercing a value to its boolean truthiness.

Question 32

In a zero-indexed array, what index holds the first element?

Accepted Answer

Index 0 holds the first element, so the last element is at index length - 1. Zero-indexing comes from offset arithmetic: element address = base + index x size, so the first element sits at offset 0.

Question 33

Which popular language uses indentation/whitespace to define code blocks?

Accepted Answer

Python uses indentation (whitespace) to define blocks instead of braces, so consistent indentation is syntactically required. This enforces readable structure, but mixing tabs and spaces can cause errors.

Question 34

Why do teams use version control like Git?

Accepted Answer

Version control records the full history of changes, lets many people work in parallel via branches without overwriting each other, supports reverting to known-good states, and shows who changed what and why. Git is distributed, so every clone holds the full history.

Question 35

What is an environment variable typically used for?

Accepted Answer

Environment variables hold configuration or secrets (API keys, database URLs, feature flags) outside the codebase, so the same build runs differently per environment (dev/staging/prod) without code changes. Keeping secrets out of source control also improves security.

Question 36

What does a unit test verify?

Accepted Answer

A unit test verifies a single small unit, often one function, in isolation with dependencies stubbed or mocked. It should be fast, deterministic, and focused on one behavior so failures are easy to localize. Unit tests form the base of the testing pyramid.

Question 37

In many C-style languages, what is the difference between = and ==?

Accepted Answer

= is assignment (store a value in a variable); == compares two values for equality. Accidentally writing = inside a condition is a classic bug. Many languages also offer === for strict, type-aware equality.

Question 38

What is the main purpose of a cache?

Accepted Answer

A cache keeps frequently or recently accessed data in faster storage (memory, CDN) so future requests skip slow recomputation or fetches, cutting latency and load. The trade-off is staleness, which is why cache invalidation is famously hard.

Question 39

In web development, what is the difference between client-side and server-side code?

Accepted Answer

Client-side code runs in the user's browser (HTML/CSS/JS), handling UI and interaction, and is visible to users. Server-side code runs on the backend, handling data, auth, and business logic, and stays private. Sensitive logic and secrets must live server-side.

Question 40

What does DNS do?

Accepted Answer

DNS (Domain Name System) resolves human-readable domain names like example.com into the IP addresses computers route to. It is a distributed, hierarchical system (root, then TLD, then authoritative servers) with caching and TTLs to reduce lookups.

Question 41

How do you typically get the number of elements in an array?

Accepted Answer

Use the language's length or size accessor: arr.length in JavaScript/Java, len(arr) in Python, arr.size in Ruby. It is typically an O(1) read of a stored count rather than a recount of every element.

Question 42

Is the integer 0 truthy or falsy in languages like Python and JavaScript?

Accepted Answer

0 is falsy in both Python and JavaScript (as are 0.0 and, in JavaScript, -0 and NaN); any non-zero number is truthy. This is why an explicit comparison like x === 0 is safer than relying on truthiness when 0 is a valid value.

Question 43

What is the difference between an inline comment and API documentation?

Accepted Answer

An inline comment clarifies a specific line or block for someone reading the source. API documentation describes a module or interface's contract (inputs, outputs, behavior) for consumers who never read the implementation, often generated from docstrings or annotations.

Question 44

What is machine code?

Accepted Answer

Machine code is the binary instruction set a specific CPU executes directly, produced from higher-level source by a compiler or assembler. It is architecture-specific (x86, ARM), which is why compiled binaries are not portable across CPU families.

Question 45

What is pseudocode?

Accepted Answer

Pseudocode describes an algorithm's logic in plain, structured language without exact syntax, so it reads across languages and is useful for design and communication. It focuses on steps and control flow while omitting implementation details like memory management.

Question 46

What happens when you access an array index that does not exist?

Accepted Answer

Behavior is language-dependent: Java and C# throw an IndexOutOfBounds exception, Python raises IndexError, JavaScript returns undefined, and C has undefined behavior that may read garbage or crash. Bounds checking is what prevents the unsafe cases.

Question 47

Why might 5 + 5 give an unexpected result in JavaScript?

Accepted Answer

JavaScript's + is overloaded: with a string operand it concatenates, so '5' + 5 yields '55' instead of 10. This comes from implicit type coercion. Convert explicitly with Number(x) or enforce strict input types to avoid it.

Question 48

What does short-circuit evaluation mean for the || operator?

Accepted Answer

With ||, evaluation stops at the first truthy operand and returns it without evaluating the rest; if all are falsy it returns the last. This enables idioms like default values (x || fallback) and guarding expensive calls.

Question 49

What does a function return if it has no explicit return statement (in JS/Python)?

Accepted Answer

A function with no explicit return, or a bare return, yields undefined in JavaScript and None in Python. Both are the language's 'no value' placeholder, so callers should not rely on a meaningful result from such functions.

Question 50

What does a linter do?

Accepted Answer

A linter statically analyzes source code without running it to flag style inconsistencies, likely bugs, and violations of coding standards, often auto-fixing some. It keeps a team consistent and catches issues early, e.g. ESLint for JavaScript or Ruff for Python.

Question 51

What is a hash collision and one common way to resolve it?

Accepted Answer

A collision is when two distinct keys hash to the same bucket. Chaining stores a list (or tree) of entries per bucket; open addressing probes for the next free slot via linear, quadratic, or double hashing. Keeping the load factor low and resizing limits collisions, preserving O(1) average operations.

Question 52

What is the average and worst-case time complexity of quicksort?

Accepted Answer

Average O(n log n), worst case O(n^2) when pivots repeatedly split off one element, e.g. already-sorted input with a naive pivot. Randomized or median-of-three pivots make the worst case unlikely. Space is O(log n) for the recursion stack, and it sorts in place.

Question 53

What does it mean for a sorting algorithm to be stable?

Accepted Answer

Stability means elements that compare equal keep their original relative order after sorting. It matters when sorting by multiple keys in passes, e.g. sort by name, then by date. Merge sort is stable; typical quicksort and heapsort are not without extra work.

Question 54

What does the ACID acronym stand for in databases?

Accepted Answer

Atomicity (transactions are all-or-nothing), Consistency (each transaction moves the database between valid states), Isolation (concurrent transactions do not interfere, per the isolation level), and Durability (committed data survives crashes). Together they guarantee reliable transaction processing.

Question 55

What is one downside of adding a database index?

Accepted Answer

An index speeds up reads such as lookups, ranges, and sorts, but every insert, update, and delete must also maintain the index, slowing writes, and it consumes extra storage and memory. Index selectively for columns used in WHERE, JOIN, and ORDER BY rather than indexing everything.

Question 56

What is the goal of database normalization?

Accepted Answer

Normalization organizes data into related tables (1NF, 2NF, 3NF) to eliminate redundancy and the insert, update, and delete anomalies it causes, with foreign keys linking tables. The trade-off is more joins, so read-heavy analytics workloads sometimes denormalize for speed.

Question 57

What is the N+1 query problem?

Accepted Answer

The N+1 problem issues one query to fetch N parents, then one extra query per parent to load related data, totaling N+1 round-trips. It often hides behind lazy-loading ORMs. Fix it with a JOIN, eager loading, or batching the child lookups into a single IN query.

Question 58

What is a deadlock?

Accepted Answer

A deadlock occurs when processes form a cycle of waits, each holding a resource the next needs. It requires four conditions: mutual exclusion, hold-and-wait, no preemption, and circular wait. Prevent it by acquiring locks in a consistent order, using timeouts, or detecting and aborting a victim.

Question 59

What is a race condition?

Accepted Answer

A race condition is when correctness depends on the unpredictable interleaving of concurrent operations on shared state, e.g. a lost update from an unsynchronized read-modify-write. Fix it with locks, atomic operations, or by avoiding shared mutable state altogether.

Question 60

What is a mutex used for?

Accepted Answer

A mutex (mutual exclusion lock) ensures only one thread holds it at a time, protecting a critical section from concurrent access while others block until it is free. Misuse causes deadlocks or contention, so hold locks briefly and acquire multiple locks in a consistent global order.

Question 61

What is the key difference between a process and a thread?

Accepted Answer

A process has its own isolated memory space, while threads within a process share that memory, making thread communication cheap but requiring synchronization. Processes are more isolated and crash-resistant but heavier to create and to communicate between via IPC.

Question 62

What is the main difference between TCP and UDP?

Accepted Answer

TCP establishes a connection and provides reliable, ordered, error-checked, flow-controlled delivery via acknowledgments and retransmits. UDP is connectionless with no delivery or ordering guarantees, so it is lighter and lower-latency, suited to streaming, gaming, and DNS where speed beats reliability.

Question 63

What does HTTPS add over HTTP?

Accepted Answer

HTTPS layers HTTP over TLS, adding confidentiality through encryption, integrity through tamper detection, and authentication of the server via certificates. The TLS handshake negotiates session keys, after which traffic is encrypted, protecting against eavesdropping and man-in-the-middle attacks.

Question 64

What is the difference between session cookies and token-based (JWT) auth?

Accepted Answer

Session auth stores state on the server and the client holds an opaque session ID in a cookie that the server looks up each request. Token auth puts signed claims in a JWT the client sends, usually in a header, so the server validates the signature without storage. JWTs scale statelessly but are hard to revoke before expiry.

Ready to practice the full interview?