The Expand-Contract Framework

Concepts covered: pyExpandContract, pyWindowTemplate

Every sliding window problem, fixed or variable width, follows the same three-step loop: expand, contract, record. The right pointer expands the window by adding an element. The left pointer contracts the window by removing elements until the constraint is satisfied. Then you record the current window as a potential answer. This expand-contract-record rhythm is the template. Memorize it. Every sliding window problem is a variation of this template. The Universal Template For fixed-width windows, step 2 simplifies to: if the window is larger than K, remove the leftmost element and advance left. No while loop needed because the window can only be one element too large. For variable-width windows, step 2 uses a while loop because the window might need to shrink by multiple elements before the

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This section is part of the Sliding Window: Beginner lesson on DataDriven, a free data engineering interview prep platform. Each section includes explanations, worked examples, and hands-on code challenges that execute in real time. SQL queries run against a live database. Python runs in a sandboxed Docker container. Data modeling problems validate against interactive schema canvases. All content is framed around what data engineering interviewers actually test at companies like Meta, Google, Amazon, Netflix, Stripe, and Databricks.

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