Question 1

What is the time complexity of common operations on arrays, hash maps, and balanced trees?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know what a list and a loop are; everything else is explained.
**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 count on this page (shifts, comparisons, tree heights) comes from running the Python code shown, and the **Verified** section quotes tha

Question 2

What is a binary search tree (BST)?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you only need to know what a variable, a loop and a function are.  
**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 t

Question 3

What are the tree traversal methods?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews; no prior knowledge of trees 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

Imagine you have just moved into

Question 4

What is dynamic programming?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews; you should be comfortable with functions, loops and 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 out

Question 5

How do you find the lowest common ancestor in a binary tree?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know what a binary tree and a recursive function are; everything else is explained.  
**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 th

Question 6

How does a hash table work?

Accepted Answer

**Target Audience:** Developers who use maps every day and want to know what happens inside — beginners welcome.
**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 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 St

Question 7

How do you validate that a binary tree is a BST?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know what a binary tree and recursion 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 output

Question 8

What is a heap / priority queue?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you need Python lists and loops; no tree knowledge assumed.  
**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 o

Question 9

How do you find the kth smallest element in a BST?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know what a binary search tree and a stack 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 ou

Question 10

How do you count unique paths in a grid?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you need nested loops and 2D arrays; no prior DP knowledge is assumed.  
**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

DATA STRUCTURES & ALGORITHMSDECODED.

Easy

What is the time complexity of common operations on arrays, hash maps, and balanced trees?

What is a binary search tree (BST)?

What are the tree traversal methods?

Medium

What is dynamic programming?

How do you find the lowest common ancestor in a binary tree?

How does a hash table work?

How do you validate that a binary tree is a BST?

What is a heap / priority queue?

How do you find the kth smallest element in a BST?

How do you count unique paths in a grid?

What is a greedy algorithm?

Hard

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

How do you find the minimum window substring?

Count of smaller numbers after self with Fenwick tree — how to coordinate compress?