Question 1

What is a graph, and how do you represent it?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — no graph knowledge needed, just arrays and loops  
**Difficulty:** Easy  
**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 run, and the output you see

Question 2

How do you count unique paths in a grid?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you need nested loops and 2D arrays; no prior DP knowledge is assumed.  
**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 run

Question 3

How do you traverse a matrix in spiral order?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you only need Python lists of lists, `range` and `while` loops.
**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 Fir

Question 4

How do you solve the Rotting Oranges problem with multi-source BFS?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you know Python lists, tuples and loops; BFS and grids-as-graphs 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. Wh

Question 5

How do you count the number of islands in a grid?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know 2-D arrays, and ideally what BFS and DFS are  
**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 run, and the

DATA STRUCTURES & ALGORITHMSDECODED.

Easy

What is a graph, and how do you represent it?

Medium

How do you count unique paths in a grid?

How do you traverse a matrix in spiral order?

How do you solve the Rotting Oranges problem with multi-source BFS?

How do you count the number of islands in a grid?