Question 1

How do you add one to a number represented as a digit array?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you only need Python lists 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 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 First

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Question 2

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

A libr

Question 3

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 4

How do you check if two strings are anagrams?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you know Python strings, loops and dicts; nothing else 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 output shown was produced by running the code with `python3` (3.13); nothing is typed in by hand.

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

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Question 5

What is a trie and when is it useful?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you need loops, classes and the idea of a tree.  
**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 se

Question 6

How do you group anagrams together?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you know Python lists, strings and dicts; nothing else assumed.
**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 running the code with `python3` (3.13); nothing is typed in by hand.

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

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Question 7

How do you find the longest substring without repeating characters?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know strings, loops and what a map is.
**Difficulty:** Medium
**Language:** Python 3

> **How to read this doc:** Every idea is explained in plain language first, then tagged `📌 Interview term:` with the exact vocabulary an interviewer expects. Every output quoted in the **Verified** section was printed by actually running the code shown with `python3`.

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

You are reading a lon

Question 8

How do you solve the word break problem?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you need loops, strings and hash sets; the DP is built up from scratch.  
**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 ru

Question 9

How do you find the longest common subsequence?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you need loops, lists and 2D lists.  
**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 the **V

Question 10

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

How do you add one to a number represented as a digit array?

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

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

How do you check if two strings are anagrams?

Medium

What is a trie and when is it useful?

How do you group anagrams together?

How do you find the longest substring without repeating characters?

How do you solve the word break problem?

How do you find the longest common subsequence?

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

How do you find the longest palindromic substring?

Hard

How does the KMP string-matching algorithm work?

How do you find the minimum window substring?

How do you serialize and deserialize a binary tree?

How do you solve the word ladder problem?

How do you compute the edit distance between two strings?

How do you solve the N-Queens problem?