Question 1

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 2

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

Picture an old car odome

Question 3

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 4

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 5

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

A sign

Question 6

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

Two

Question 7

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 8

Implement Trie for autocomplete — how do you handle wildcards (.)?

Accepted Answer

**Target Audience:** Developers who know what a trie is and want the classic design question — you need recursion and basic classes.  
**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, a

Question 9

Decode ways DP: how do you handle 0 and 06 edge cases?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you need loops, strings and characters; the DP idea 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 actua

Question 10

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

A

DATA STRUCTURES & ALGORITHMSDECODED.

Easy

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

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 for valid/balanced parentheses?

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?

Implement Trie for autocomplete — how do you handle wildcards (.)?

Decode ways DP: how do you handle 0 and 06 edge cases?

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?

Hard

How does the KMP string-matching algorithm work?

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

Alien Dictionary: given sorted words in an alien language, how do you derive the character order?

How do you compute the edit distance between two strings?

How do you solve the N-Queens problem?