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

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 3

How do you merge two sorted linked lists?

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

Two te

Question 4

What is space complexity, and how much stack space does recursion use?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know what a function call and a loop are.
**Difficulty:** Easy
**Language:** Python 3

> **How to read this doc:** Each idea is explained in plain words first, then tagged with `📌 Interview term:` so you also learn the exact vocabulary an interviewer expects. Every depth, count and error message on this page comes from running the Python code shown; the **Verified** section quotes that output. This page assumes you know

Question 5

What is recursion and what is a base case?

Accepted Answer

**Target Audience:** Complete beginners preparing for coding interviews — you only need to know what a function is.
**Difficulty:** Easy
**Language:** Python 3

> **How to read this doc:** Each idea is explained in plain words first, then tagged with `📌 Interview term:` so you also learn the exact vocabulary an interviewer expects. Every trace, count and error message on this page comes from running the Python code shown, and the **Verified** section quotes that output.

## 1. Why This Even Mat

Question 6

What is a binary search tree (BST)?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you only need to know what a variable, a loop and a function are.  
**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 t

Question 7

How do you check if a root-to-leaf path sums to a target?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews; you should know what a binary tree is.
**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 words an interviewer expects. Every code block in this Python version was run, and the **Verified** section quotes the real output.

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

You are on a hike with a fixed

Question 8

How do you invert a binary tree?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews; you should know what a binary tree is.
**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 words an interviewer expects. Every code block in this Python version was run, and the **Verified** section quotes the real output.

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

Hold a printed organisation cha

Question 9

How should you approach a DSA problem in a coding interview?

Accepted Answer

**Target Audience:** Anyone preparing for their first coding interviews — no prior algorithms 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 Stor

Question 10

How do you check if a binary tree is symmetric?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews; you should know what a binary tree is.
**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 words an interviewer expects. Every code block in this Python version was run, and the **Verified** section quotes the real output.

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

Fold a sheet of paper in half,

DATA STRUCTURES & ALGORITHMSDECODED.

Easy

How do you reverse a linked list?

How do you solve the climbing stairs problem?

How do you merge two sorted linked lists?

What is space complexity, and how much stack space does recursion use?

What is recursion and what is a base case?

What is a binary search tree (BST)?

How do you check if a root-to-leaf path sums to a target?

How do you invert a binary tree?

Medium

What is topological sort and when is it used?

What is dynamic programming?

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

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

How does Quickselect find the kth smallest/largest element?

How do you check if a binary tree is balanced?

How do you detect a cycle in a directed graph?

How do you check if a graph is bipartite?

Hard

How do you serialize and deserialize a binary tree?

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

What is a segment tree and when do you need one?

Burst Balloons: why does DP over intervals (not balloons) work?

How do you solve the N-Queens problem?

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