Question 1

What is a queue and a stack, and when do you use each?

Accepted Answer

Two of the simplest data structures, distinguished entirely by **which element you get back**: a stack returns the most recently added, a queue returns the oldest. That single difference determines an enormous range of behaviour.

**A stack is last in, first out.** Push onto the top, pop from the top. Think of a pile of plates.

**Where stacks appear:** the **call stack**, holding function frames, which is why the data structure and the error share a name; undo history, where the most recent action is reversed first; expression evaluation and bracket matching; **depth-first traversal** of trees and graphs; and backtracking algorithms.

**A queue is first in, first out.** Enqueue at the back, dequeue from the front. Think of a line of people.

**Where queues appear:** task and job processing, where fairness matters; **breadth-first traversal**, which finds shortest paths in unweighted graphs; buffering between a fast producer and a slow consumer; request handling; and print and message queues.

**The traversal point is the clearest illustration.** Depth-first and breadth-first search are **the same algorithm** with the same structure — the only difference is whether pending nodes are held in a stack or a queue. Swapping one for the other changes the entire character of the search.

**The variants worth knowing:**

**Deque**, allowing insertion and removal at both ends, which can act as either.

**Priority queue**, returning the highest-priority element rather than the oldest — implemented with a heap, and the basis of scheduling and of shortest-path algorithms.

**Circular buffer**, a fixed-size queue that overwrites the oldest entry when full, used in logging and audio processing where bounded memory matters more than completeness.

**What to watch for in practice:** **unbounded queues**, which convert a throughput problem into an out-of-memory crash and hide the fact that consumers cannot keep up; and deep recursion, which is a stack that will eventually overflow.

**In most languages you will not implement either** — a dynamic array serves as a stack, and standard libraries provide efficient deques and priority queues.

Question 2

What is recursion, and when should you use it?

Accepted Answer

A function that calls itself, solving a problem by reducing it to a smaller version of the same problem until it reaches a case simple enough to answer directly. The two required parts. A base case,

Question 3

What is the difference between the stack and the heap?

Accepted Answer

Both are regions of memory a program uses, with fundamentally different allocation strategies and lifetimes. The stack holds function call frames. Each call pushes a frame containing parameters,

Question 4

What is a message queue and when do you need one?

Accepted Answer

A message queue sits between a producer of work and a consumer of it, holding messages until they can be processed. It converts a synchronous call into an asynchronous one. What it gives you:

Question 5

How do you read a stack trace?

Accepted Answer

A stack trace is a snapshot of the chain of function calls in progress when something went wrong — read correctly it usually identifies the problem in under a minute, and read incorrectly it looks

Question 6

When should you use an array rather than a linked list?

Accepted Answer

Almost always — and the textbook answer, which favours linked lists for insertion and deletion, is misleading on modern hardware because it ignores how memory actually behaves. The theoretical

Question 7

What is the difference between a process and a thread?

Accepted Answer

A process is an independent running program with its own memory space. A thread is a unit of execution inside a process, sharing that process's memory with its sibling threads. What a process owns:

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