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 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 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

How do you check for valid/balanced parentheses?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews; no prior knowledge of stacks 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 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

Think of a stack of plates in a

Question 6

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 7

How do you compute the GCD of two numbers?

Accepted Answer

**Target Audience:** Developers preparing for coding interviews who know loops and the modulo operator in Python; the math is explained from scratch.  
**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

Question 8

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 9

What are the tree traversal methods?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews; no prior knowledge of trees 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 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

Imagine you have just moved into

Question 10

How do you find the maximum depth of 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

Picture a family tree drawn on

DATA STRUCTURES & ALGORITHMSDECODED.

Easy

How do you reverse a linked list?

What is the difference between a stack and a queue?

How do you merge two sorted linked lists?

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

How do you check for valid/balanced parentheses?

What is recursion and what is a base case?

How do you compute the GCD of two numbers?

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

Medium

How do you add two numbers stored as linked lists?

How do you implement a queue using two stacks?

How do you check if a binary tree is balanced?

How do you find the next greater element for every item (Daily Temperatures)?

How do you validate that a binary tree is a BST?

What is a monotonic stack and when do you use it?

How do you count connected components in an undirected graph?

How do you find the kth smallest element in a BST?

Hard

How do you solve the trapping rain water problem?

How do you find the largest rectangle in a histogram?

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