Question 1

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 2

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

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Question 3

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 4

How do you find the longest common prefix of an array of strings?

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

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Question 5

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 6

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

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Question 7

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 8

How do you check if a string is a valid palindrome?

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

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Question 9

How do you generate all subsets of an array?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know arrays and basic recursion  
**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 output you see in

Question 10

How do you solve the Combination Sum problem?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know recursion and the subsets pattern  
**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 output you

DATA STRUCTURES & ALGORITHMSDECODED.

Easy

What is the prefix sum technique?

What is the fast and slow pointer technique?

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

How do you find the longest common prefix of an array of strings?

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

How do you invert a binary tree?

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

How do you check if a string is a valid palindrome?

Medium

How do you generate all subsets of an array?

How do you solve the Combination Sum problem?

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

Find the kth largest element in an array

How do you generate all permutations of an array?

How do you search a sorted 2D matrix?

How do you solve the Gas Station problem?

How do merge sort and quicksort work?

Hard

How does the KMP string-matching algorithm work?

How does the Floyd-Warshall algorithm work?

How do you find the largest rectangle in a histogram?

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

How do you solve the N-Queens problem?

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

What is bitmask dynamic programming?