Sliding Window Interview Questions & Solutions

The sliding window pattern is the go-to for problems about contiguous subarrays or substrings — "longest," "shortest," or "containing" some condition. Instead of re-examining every possible range (the O(n²) trap), you maintain a window with two pointers, expanding the right edge to include new elements and contracting the left edge when a constraint breaks. The result is a single clean O(n) pass.

These solutions separate the two flavors clearly: the fixed-size window (averages and maximums over every length-k range) and the harder variable-size window (longest substring without repeating characters, minimum window substring, longest substring with at most k distinct). The variable window is where candidates stumble, so each walkthrough is explicit about the invariant the window maintains and the exact condition that triggers a contraction — usually tracked with a hash map of counts. Sliding window pairs naturally with hashing and two pointers, and once the "grow right, shrink left" rhythm is automatic, a large family of string interview questions becomes routine.