Top 18 Algorithm Interview Questions Explained: From Sorting to Recursion

Algorithms are a fundamental yet complex discipline that every programmer studies, and these 18 questions are the most frequently encountered in interviews, providing essential answers for algorithm‑focused engineers and interested developers.

1) Please briefly explain what an algorithm is?

An algorithm is a well‑defined computational process that takes some input values and produces corresponding output values; essentially, it is a series of steps that transform input into output.

2) Explain what quicksort is?

Quicksort sorts a list by partitioning it around a pivot element, dividing the list into three parts: elements less than the pivot, the pivot itself, and elements greater than the pivot.

3) Explain algorithm time complexity?

Time complexity describes the total time required for a program to run, usually expressed with Big‑O notation.

4) What symbols are used for time‑complexity notation?

The symbols include:

Big Oh (O) – upper bound

Big Omega (Ω) – lower bound

Big Theta (Θ) – tight bound

Little Oh (o) – strictly smaller

Little Omega (ω) – strictly larger

5) How does binary search work?

Binary search repeatedly compares the target value with the middle element of a sorted array, narrowing the search interval until the value is found.

6) Can binary search be applied to linked lists?

Because random access is not feasible in linked lists, binary search cannot achieve O(1) middle‑element access; it is generally unsuitable for standard linked lists, though it can be used on sorted arrays or structures that allow indexed access.

7) What is heap sort?

Heap sort is an improved selection sort that treats the input as a heap, repeatedly extracting the smallest element and moving it to the sorted region.

8) What is a skip list?

A skip list is a data structure that enables fast search, insertion, and deletion in an ordered list by maintaining multiple levels of linked lists with “express lanes.”

9) What is the space complexity of insertion sort?

Insertion sort is an in‑place algorithm requiring only O(1) additional storage.

10) What are hash algorithms and what are they used for?

Hash algorithms map arbitrary‑length strings to fixed‑length values and are used for password verification, data integrity, and various cryptographic purposes.

11) How to detect a cycle in a linked list?

By using two pointers (slow and fast); if they ever point to the same node, the list contains a cycle.

12) How do encryption algorithms work?

Encryption transforms plaintext into ciphertext using a key; larger keys increase the number of possible ciphertext patterns. Most algorithms operate on fixed‑size blocks (64‑128 bits) or use stream methods.

13) List some common encryption algorithms.

3‑way

Blowfish

CAST

CMEA

GOST

DES and Triple DES

IDEA

LOKI, etc.

14) Difference between best‑case and worst‑case for an algorithm?

Best case occurs when the input arrangement yields the smallest possible running time (e.g., binary search finding the target at the middle). Worst case occurs with the most unfavorable input (e.g., quicksort choosing the smallest or largest element as pivot, degrading to O(n²)).

15) What is radix sort?

Radix sort, also known as bucket sort, distributes elements into buckets based on digit values and processes them sequentially; it is a linear‑time sorting algorithm.

16) What is a recursive algorithm?

A recursive algorithm solves a problem by breaking it into smaller sub‑problems until they become trivial, typically involving a function that calls itself.

17) What are the three laws of recursion?

All recursive algorithms must follow three rules: they must have a base case, they must progress toward that base case, and they must invoke themselves.

18) What is bubble sort?

Bubble sort repeatedly compares adjacent elements and swaps them if they are in the wrong order, causing larger elements to “bubble” to the end of the list until the list is sorted.