Question 1

How do you implement a queue using two stacks?

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

A bakery has a tall, narrow tray ra

Question 2

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 3

How do you design a min stack with O(1) getMin?

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

Picture a pile of exam papers on a

Question 4

Find median from data stream with two heaps — invariant and rebalancing?

Accepted Answer

**Target Audience:** Developers who have seen the two-heap median solution in Python and want to explain why it is correct.  
**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 5

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 6

How do you serialize and deserialize a binary tree?

Accepted Answer

**Target Audience:** Intermediate interview candidates — you should know binary trees, recursion and BFS with a queue.  
**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 7

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

Accepted Answer

**Target Audience:** Developers preparing for coding interviews who know classes and references in Python and want to justify the LRU design, not just recite it.  
**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 th

Question 8

How do you find the median from a data stream?

Accepted Answer

**Target Audience:** Developers preparing for coding interviews who know arrays and basic classes in Python; heaps are explained as we go.  
**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 ru

Question 9

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

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

DATA STRUCTURES & ALGORITHMSDECODED.

Medium

How do you implement a queue using two stacks?

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

How do you design a min stack with O(1) getMin?

Find median from data stream with two heaps — invariant and rebalancing?

Hard

How do you design an LRU cache?

How do you serialize and deserialize a binary tree?

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?

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