Question 1

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 2

What is a trie and when is it useful?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you need loops, classes and the idea of a tree.  
**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 se

Question 3

Implement Trie for autocomplete — how do you handle wildcards (.)?

Accepted Answer

**Target Audience:** Developers who know what a trie is and want the classic design question — you need recursion and basic classes.  
**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, a

Question 4

How do you find the maximum product subarray?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you need loops, arrays and the idea of a running best value.  
**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

Question 5

Merge k sorted lists: heap vs divide-and-conquer — which is better for n=10k?

Accepted Answer

**Target Audience:** Developers who know basic arrays and loops in Python and want to reason about algorithm trade-offs, not just Big-O.  
**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 ru

Question 6

Word Ladder: shortest transformation via BFS — how do you build the wildcard graph?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you know Python dicts and strings, and ideally have read the basic Word Ladder BFS; everything else is explained from scratch.
**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 output shown was produced by actually running the code with `python3` (3.13); noth

Question 7

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

Accepted Answer

**Target Audience:** Developers who know the two-heap running median and want to extend it to a moving window in Python.  
**Difficulty:** Hard  
**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

How do you design an LRU cache?

Accepted Answer

**Target Audience:** Developers preparing for coding and system design interviews who know maps and classes in Python.  
**Difficulty:** Hard  
**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 yo

Question 9

How does Dijkstra's algorithm work?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — no graph-algorithm background assumed
**Difficulty:** Hard
**Language:** Python 3