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Sliding Window Interview Questions — Pattern Guide

A pattern guide to sliding window problems: fixed vs variable windows, the two-pointer invariant, and the problems that map to each.

Updated 2026-09MediumSliding Window

When to reach for a sliding window

Sliding window applies when you need the best contiguous subarray or substring satisfying a constraint. The window moves monotonically, giving O(n) instead of O(n²) nested-loop solutions.

Fixed vs variable windows

Fixed window: size k is given; maintain a running sum/count and slide one step at a time.
Variable window: expand the right edge, shrink the left edge when a constraint is violated, track the best valid window.
A frequency map or counter tracks what is inside the window.

Canonical problems

Maximum sum subarray of size k (fixed).
Longest substring without repeating characters (variable).
Minimum window substring containing all target characters (variable).
Longest substring with at most k distinct characters (variable).
Permutation in string (fixed-size match check).

How to structure the solution

Define the window state (sum, counts, distinct count).
Expand right; update the state.
While the window is invalid, shrink from the left and update the state.
When the window is valid, record the answer.

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This problem at 10 companies

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This problem for 9 roles

Senior Frontend Engineer Senior Backend Engineer Staff Engineer Full Stack Engineer AI Engineer AI/FDE (Forward Deployed Engineer)Machine Learning Engineer Engineering Manager New Grad Software Engineer