How to Master the Sliding Window Pattern

The sliding window technique maintains a "window" of elements in an array or string and expands or shrinks it based on a constraint. Instead of checking every possible subarray in O(n²), the window lets each element enter and exit at most once, giving O(n) time. It is one of the most common patterns in FAANG coding interviews.

Use sliding window when you need a contiguous subarray or substring satisfying some constraint (longest, shortest, count). Use two pointers when the array is sorted and you are looking for pairs or triplets. Sliding window is a specialized form of two pointers where both pointers move in the same direction.

The most frequently asked sliding window problems are: Longest Substring Without Repeating Characters (LC 3), Minimum Window Substring (LC 76), Longest Repeating Character Replacement (LC 424), Permutation in String (LC 567), and Sliding Window Maximum (LC 239). Minimum Window Substring is the hardest and most commonly asked at Google and Meta.

Look for these keywords: "contiguous subarray", "substring", "longest/shortest with constraint", "at most k distinct elements", or "minimum window containing". If the problem asks about a range of consecutive elements with some sum or count constraint, sliding window is likely the right approach.

O(n) for both fixed and variable-size windows. Each element enters the window once (right pointer moves right) and exits at most once (left pointer moves right), so the total operations are bounded by 2n, which is O(n). Space complexity is O(k) where k is the size of the window or the number of distinct elements tracked.