Question 1

Number of islands with DFS vs BFS vs Union-Find — trade-offs?

Accepted Answer

**Target Audience:** Candidates who can already solve number of islands once and want to compare the three standard approaches  
**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 2

Trapping Rain Water with two pointers — why does leftMax/rightMax work in O(1) space?

Accepted Answer

**Target Audience:** Intermediate — you have seen the trapping rain water problem and can read Python; the proof is explained from scratch.  
**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

Question 3

Longest consecutive sequence in O(n) with hash set — why not sort?

Accepted Answer

**Target Audience:** Intermediate candidates — you should know arrays and what a hash set does; Big-O basics help.
**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 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

Question 4

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 5

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

Accepted Answer

**Target Audience:** Intermediate candidates who know the sliding window idea and want to explain the counters precisely; every term is defined.
**Difficulty:** Hard
**Language:** Python 3

> **How to read this doc:** Every idea is explained in plain language first, then tagged `📌 Interview term:` with the exact vocabulary an interviewer expects. Every output quoted in the **Verified** section was printed by actually running the code shown with `python3`.

## 1. Why This Even Matters — A Story

Question 6

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