Question 1

What is the fast and slow pointer technique?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — no prior algorithms knowledge assumed
**Difficulty:** Easy
**Language:** Python 3

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. All code is Python, and every output block in the **Verified** section was copied from a real run of the examples on this page.

## 1. Why This Even Matters — A Story First

A tort

Question 2

How do you find the intersection node of two linked lists?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know what a linked list node is; everything else is explained
**Difficulty:** Easy
**Language:** Python 3

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. All code is Python, and every output block in the **Verified** section was copied from a real run of the examples on this page.

## 1. Why This E

Question 3

How do you find a duplicate in an array?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you know Python lists, loops and sets; nothing else assumed.
**Difficulty:** Easy (the general version); the O(1)-space Floyd variant is often graded Medium
**Language:** Python 3

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Every output shown was produced by running the code with `python3` (3.13); nothin

Question 4

What is topological sort and when is it used?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — no prior algorithms knowledge assumed
**Difficulty:** Medium
**Language:** Python 3

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. All code is Python, and every output block in the **Verified** section was copied from a real run of the examples on this page.

## 1. Why This Even Matters — A Story First

Thin

Question 5

How do you solve the course schedule (prerequisites) problem?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — no prior algorithms knowledge assumed
**Difficulty:** Medium
**Language:** Python 3

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. All code is Python, and every output block in the **Verified** section was copied from a real run of the examples on this page.

## 1. Why This Even Matters — A Story First

You

Question 6

How do you detect a cycle in a directed graph?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — no prior algorithms knowledge assumed
**Difficulty:** Medium
**Language:** Python 3

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. All code is Python, and every output block in the **Verified** section was copied from a real run of the examples on this page.

## 1. Why This Even Matters — A Story First

Pict

Question 7

How do you check if a graph is bipartite?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you know Python lists and loops; graphs and colouring are explained from scratch.
**Difficulty:** Medium
**Language:** Python 3

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Every output shown was produced by actually running the code with `python3` (3.13); nothing is typed in by hand.

## 1. Why This Even

Question 8

How do you rotate an array by k positions?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you only need Python lists, loops and the `%` operator.
**Difficulty:** Medium
**Language:** Python 3

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the word an interviewer expects. Every program here was run with `python3`, and the output shown in the **Verified** section is copied from that run.

## 1. Why This Even Matters — A Story First

Seve

Question 9

How do you count connected components in an undirected graph?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know what an adjacency list is and roughly how BFS or DFS work  
**Difficulty:** Medium  
**Language:** Python 3

> **How to read this doc:** every idea is explained in plain words first. When a word appears that an interviewer will expect you to know, it is tagged `📌 Interview term:` with a one-line definition, and every tagged term is collected in the Quick Glossary at the end. Every program on this page was actually

Question 10

How do you find the node where a linked list cycle begins?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know what a linked list node is; everything else is explained
**Difficulty:** Medium
**Language:** Python 3

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. All code is Python, and every output block in the **Verified** section was copied from a real run of the examples on this page.

## 1. Why This

DATA STRUCTURES & ALGORITHMSDECODED.

Easy

What is the fast and slow pointer technique?

How do you find the intersection node of two linked lists?

How do you find a duplicate in an array?

Medium

What is topological sort and when is it used?

How do you solve the course schedule (prerequisites) problem?

How do you detect a cycle in a directed graph?

How do you check if a graph is bipartite?

How do you rotate an array by k positions?

How do you count connected components in an undirected graph?

How do you find the node where a linked list cycle begins?

What is the difference between BFS and DFS?

Hard

How does the Floyd-Warshall algorithm work?

How does the Bellman-Ford algorithm work?

Longest increasing path in matrix — why DFS + memo beats brute force?

Alien Dictionary: given sorted words in an alien language, how do you derive the character order?

How do you find strongly connected components in a directed graph?