Question 1

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

Accepted Answer

**Target Audience:** Candidates who already know Kadane's algorithm and want to explain why it works and how it compares.
**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 y

Question 2

Course Schedule: detect cycle with topological sort (Kahn vs DFS)?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — no prior algorithms knowledge assumed
**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 Even Matters — A Story First

Two

Question 3

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 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

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

Accepted Answer

**Target Audience:** Developers who know arrays, sorting and binary search and want the classic Fenwick tree problem.  
**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 you

Question 6

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 7

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

Accepted Answer

**Target Audience:** Intermediate interview candidates — you should know binary search and what a sorted array is; every other idea is explained here.
**Difficulty:** Hard
**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, step count and ordering claim below was produced by actually running the Python programs in the **Verified** section.

DATA STRUCTURES & ALGORITHMSDECODED.

Medium