Question 1

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 2

How do you move all zeroes to the end of an array in place?

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 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 4

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

Question 5

How do you generate all permutations 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 6

How do you rotate an N×N matrix by 90 degrees in place?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you only need Python lists of lists and nested loops.
**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 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

You ar

DATA STRUCTURES & ALGORITHMSDECODED.

Easy

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

How do you move all zeroes to the end of an array in place?

How do you merge two sorted arrays?

Medium

How does Quickselect find the kth smallest/largest element?

How do you generate all permutations of an array?

How do you rotate an N×N matrix by 90 degrees in place?