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

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

**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 First A teacher is checking whether a group of students can form a team: the rule is **two** people who speak French, **one** who codes and **one** who draws. As students walk into the room one by one, she could re-read the whole rule list after every arrival — or she could keep a single tally on the board: "requirements met: 0 of 3". When the second French speaker walks in, the French requirement flips from unmet to met, so the tally goes to 1 of 3. A third French speaker changes nothing; the

The problem

Question presented to candidate: "In the minimum window substring problem, people keep a 'need' map and a 'have' counter. Explain exactly what each one holds, when each counter changes, and why that makes the window check constant time. Then write the solution in Python."

What a strong answer should cover:

need = counts of t; window = counts in the current slice; required = distinct characters of t; have = distinct characters whose window count is at least their need.
The window is valid exactly when have == required, so the check is O(1) instead of O(k).
have changes only on threshold crossings: +1 when a count reaches its need after adding, -1 when it drops below after removing; spare copies change nothing.
The equivalent missing counter over total copies, and the two classic bugs (>= on add, <= on remove).
Python specifics: Counter(t) for need and len(need) for required; ch in need before comparing, because characters outside t must not touch have; need.get(c, 0) for the missing-counter variant.

Clarifying questions expected:

"Do duplicate characters in t have to be matched with multiplicity?" — this is what makes counts necessary.

Code / implementation expected: Yes — the min-window function in Python, explaining each counter update as it is written.

sliding window
string
Meta

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Solution

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 First

A teacher is checking whether a group of students can form a team: the rule is two people who speak French, one who codes and one who draws. As students walk into the room one by one, she could re-read the whole rule list after every arrival — or she could keep a single tally on the board: "requirements met: 0 of 3". When the second French speaker walks in, the French requiremen

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7 min read · 12 sections

Python code

Edit the code and run it here. Each printed line with an expected value becomes a test.

PythonTrace every need/have change on ADOBECODEBANC / ABC

Practice

Pythonhave vs missing counters, and the two classic off-by-one bugs

Practice

Discussion

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