Question 1

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 2

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

Picture an old car odome

Question 3

How do you compute the GCD of two numbers?

Accepted Answer

**Target Audience:** Developers preparing for coding interviews who know loops and the modulo operator in Python; the math is 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

Question 4

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 5

How do you add two numbers stored as linked lists?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know what a linked list node is; everything else is explained
**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

Question 6

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

Question 7

How does the Sieve of Eratosthenes work?

Accepted Answer

**Target Audience:** Developers preparing for coding interviews who know arrays and loops in Python; primes and the math are explained from scratch.  
**Difficulty:** Medium  
**Language:** Python 3