Question 1

How do you reverse a linked list?

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

Pictur

Question 2

What is the prefix sum technique?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know arrays and loops.
**Difficulty:** Easy
**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 exact words an interviewer expects. Every output shown in the **Verified** section was produced by actually running the Python code with `python3 main.py`; nothing is quoted from memory.

## 1. Why This Even Matters — A

Question 3

How does binary search work and what are its requirements?

Accepted Answer

# How Binary Search Works and What It Requires (Python)

**Target Audience:** Beginners preparing for coding interviews; you only need to know arrays and loops.
**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 was produced by actually running the programs (Python 3.13.16); nothing is quot

Question 4

How do you solve the climbing stairs problem?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews; this is usually the first dynamic programming problem people meet.
**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 o

Question 5

What is the difference between an array and a linked list?

Accepted Answer

**Target Audience:** Beginners learning data structures — no prior knowledge assumed.
**Difficulty:** Easy
**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 exact words an interviewer expects. Every program in this doc was run with `python3 main.py`, and the output quoted in the **Verified** section is copied from that run.

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

Picture two ways o

Question 6

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 7

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 8

What is the difference between a stack and a queue?

Accepted Answer

**Target Audience:** Beginners learning data structures — no prior knowledge assumed.
**Difficulty:** Easy
**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 exact words an interviewer expects. Every program in this doc was run with `python3 main.py`, and the output quoted in the **Verified** section is copied from that run.

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

In a restaurant ki

Question 9

What is Big-O notation?

Accepted Answer

**Target Audience:** Complete beginners preparing for coding interviews — no maths beyond multiplication assumed.
**Difficulty:** Easy
**Language:** Python 3

> **How to read this doc:** Every idea is explained in plain words first, then tagged with `📌 Interview term:` so you also learn the exact vocabulary an interviewer expects. Every number on this page (step counts, comparisons, overflow values) comes from actually running the Python code shown, and the **Verified** section quotes that outp

Question 10

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

DATA STRUCTURES & ALGORITHMSDECODED.

Easy

How do you reverse a linked list?

What is the prefix sum technique?

How does binary search work and what are its requirements?

How do you solve the climbing stairs problem?

What is the difference between an array and a linked list?

What is the fast and slow pointer technique?

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

What is the difference between a stack and a queue?

Medium

What is topological sort and when is it used?

What is dynamic programming?

What is a trie and when is it useful?

How do you find the lowest common ancestor in a binary tree?

How do you find the k closest points to the origin?

How do you solve the 0/1 knapsack problem?

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

How do you add two numbers stored as linked lists?

Hard

Sliding window median: how do you maintain median with two heaps as the window slides?

How do you design an LRU cache?

How does the KMP string-matching algorithm work?

How do you merge k sorted lists?

Minimum window substring: how does the sliding window with need/have counters work?

How do you find the minimum window substring?

How does the Floyd-Warshall algorithm work?

How do you serialize and deserialize a binary tree?