Question 1

What is the difference between a stack and a queue?

Accepted Answer

**Target Audience:** Beginners learning data structures — no prior 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 Story First

In a restaurant ki

Question 2

How do you find the best time to buy and sell a stock?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you only need to know loops and variables in Python.  
**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

Question 3

How do you merge two sorted linked lists?

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

Two te

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 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 6

What is topological sort and when is it used?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — no prior algorithms knowledge 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 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

Thin

Question 7

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 8

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 9

Find the kth largest element in an array

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know arrays and sorting; heaps and Quickselect are summarised and linked.
**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 10

What is a monotonic stack and when do you use it?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews; you should know what a stack is.
**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 code block in this Python version was run, and the **Verified** section quotes the real output.

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

Imagine people standing in a line a

DATA STRUCTURES & ALGORITHMSDECODED.

Easy

What is the difference between a stack and a queue?

How do you find the best time to buy and sell a stock?

How do you merge two sorted linked lists?

What is a stable sorting algorithm?

How do you merge two sorted arrays?

Medium

What is topological sort and when is it used?

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

How does Quickselect find the kth smallest/largest element?

Find the kth largest element in an array

What is a monotonic stack and when do you use it?

What is a heap / priority queue?

When would you use BFS over Dijkstra for shortest paths?

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

Hard

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

How do you merge k sorted lists?

What is the difference between Kruskal's and Prim's MST algorithms?

Design an LRU Cache with O(1) get and put — why doubly linked list + hash map?

How do you find the median from a data stream?

How does Dijkstra's algorithm work?

Design Twitter feed: how do you fan-out with heap + hash for timeline?