Question 1

How do you find the longest substring without repeating characters?

Accepted Answer

**Target Audience:** Beginners preparing for coding interviews — you should know strings, loops and what a map is.
**Difficulty:** Medium
**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 First

You are reading a lon

Question 2

How do you count subarrays that sum to k?

Accepted Answer

**Target Audience:** Intermediate candidates — you should know arrays and hash maps; prefix sums are explained here and linked.
**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.

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Question 3

What is the sliding window technique?

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

You

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

How do you find the minimum window substring?

Accepted Answer

**Target Audience:** Intermediate candidates — you should know strings, maps and the basic sliding window idea; every term is still defined here.
**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 7

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

DATA STRUCTURES & ALGORITHMSDECODED.

Medium

How do you find the longest substring without repeating characters?

How do you count subarrays that sum to k?

What is the sliding window technique?

Hard

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

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

How do you find the minimum window substring?

How do you find the median from a data stream?