Question 1

What is Big-O notation?

Accepted Answer

**Target Audience:** Complete beginners preparing for coding interviews — no maths beyond multiplication assumed.
**Difficulty:** Easy
**Language:** Python 3

> **How to read this doc:** Every idea is explained in plain words first, then tagged with `📌 Interview term:` so you also learn the exact vocabulary an interviewer expects. Every number on this page (step counts, comparisons, overflow values) comes from actually running the Python code shown, and the **Verified** section quotes that outp

Question 2

How do you find the missing number in 0..n?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know what an array and a loop are; nothing else is 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 output shown in the **Verified** section was produced by actually running the Python code with `python3 main.py`; nothing is quoted from memo

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

What is a stable sorting algorithm?

Accepted Answer

**Target Audience:** Beginners who know what sorting is and want to understand the guarantees behind it.
**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

Question 5

How do you find the single number where every other element appears twice?

Accepted Answer

**Target Audience:** Developers preparing for coding interviews who know arrays and loops in Python; binary and XOR are 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 actual

Question 6

How do you merge two sorted arrays?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you only need to know 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

Two teachers hav

Question 7

How do you find the majority element?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you know Python lists and loops; 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

A crowd of s

Question 8

How do you solve the Two Sum problem efficiently?

Accepted Answer

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

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

Question 9

How do you find the k closest points to the origin?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you need Python lists, tuples and loops; heaps are explained on the linked page.  
**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 ac

Question 10

How does Quickselect find the kth smallest/largest element?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know arrays and loops; partitioning and pivots are explained from scratch.
**Difficulty:** Medium
**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, step count and ordering claim below was produced by actually running the Python programs in the **Verified** section

DATA STRUCTURES & ALGORITHMSDECODED.

Easy

What is Big-O notation?

How do you find the missing number in 0..n?

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

What is a stable sorting algorithm?

How do you find the single number where every other element appears twice?

How do you merge two sorted arrays?

How do you find the majority element?

How do you solve the Two Sum problem efficiently?

Medium

How do you find the k closest points to the origin?

How does Quickselect find the kth smallest/largest element?

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

How do you sort an array of 0s, 1s, and 2s?

Find the kth largest element in an array

How do you merge overlapping intervals?

How do you find the minimum number of meeting rooms required?

How do you solve the 3Sum problem?

Hard

Sliding window median: how do you maintain median with two heaps as the window slides?

How do you find the median from a data stream?