Question 1

What is the prefix sum technique?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know arrays 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 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 memory.

## 1. Why This Even Matters — A

Question 2

What is the difference between an array and a linked list?

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

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Question 3

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 4

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 5

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

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Question 6

How do you find the middle of a linked list?

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

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Question 7

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 8

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

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Question 9

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

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Question 10

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

DATA STRUCTURES & ALGORITHMSDECODED.

Easy

What is the prefix sum technique?

What is the difference between an array and a linked list?

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

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

How do you add one to a number represented as a digit array?

How do you find the middle of a linked list?

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

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

Medium

What is a trie and when is it useful?

How does Quickselect find the kth smallest/largest element?

How do you check if a graph is bipartite?

How do you generate all subsets of an array?

Maximum subarray (Kadane) vs divide-and-conquer — proof of correctness?

How do you search in a rotated sorted array?

How do you find the next greater element for every item (Daily Temperatures)?

How do you rotate an array by k positions?

Hard

How does the KMP string-matching algorithm work?

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

How do you solve the trapping rain water problem?

How do you find the largest rectangle in a histogram?

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

What is a segment tree and when do you need one?

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

Median of two sorted arrays in O(log(min(m,n))) — how does the partition trick work?