What is the sliding window technique?

Q: What is the sliding window technique?

**Target Audience:** Beginners preparing for coding interviews — no prior algorithms knowledge assumed **Difficulty:** Medium **Language:** Python 3 > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. All code is Python, and every output block in the **Verified** section was copied from a real run of the examples on this page. ## 1. Why This Even Matters — A Story First You are on a train, looking out of a window that always shows exactly three houses. You want to know the moment the three visible houses have the most lit windows in total. The slow way: every time the view changes, count the lit windows of all three houses again from scratch. The smart way: when the train moves one house along, one house disappears on the left and one appears on the right. Take the old total, subtract the house that left, add the house that arrived. One subtraction and one addition

The problem

Question presented to candidate: "What is the sliding window technique? Show me, in Python, the largest sum of any k consecutive numbers, and then a version where the window size is not fixed."

What a strong answer should cover:

A window is a contiguous range [left, right]; you update a running summary as it moves instead of recomputing it.
Fixed window (subtract the leaving element, add the entering one) versus variable window (expand right, shrink left by a rule).
O(n) time because each pointer moves forward at most n times (amortized), versus O(n·k) or O(n²) recomputing.
When it fails: the rule must be monotonic, so sums with negative numbers need prefix sums instead.
Python specifics: compute sum(arr[:k]) once, never sum(arr[i:i+k]) per step (each slice copies k items); collections.Counter or defaultdict(int) for character windows; float('inf') as a "no answer yet" sentinel; ints cannot overflow.

Clarifying questions expected:

"Can the numbers be negative or zero?" — decides whether shrinking is valid.
"What should I return if no window qualifies, or if k is larger than the array?"

Code / implementation expected: Yes — max_window_sum(arr, k) for the fixed window and one variable-window function such as min_subarray_len(target, nums), in Python.

sliding-window

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners preparing for coding interviews — no prior algorithms knowledge assumed Difficulty: Medium Language: Python 3

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact word an interviewer expects. All code is Python, and every output block in the Verified section was copied from a real run of the examples on this page.

1. Why This Even Matters — A Story First

You are on a train, looking out of a window that always shows exactly three houses. You want to know the moment the three visible houses have the most lit windows in total. The slow way: every time the view changes, count the lit windows of all three houses again from scratch. The smart way: when the train moves one house along, one house disappears on the left and one appears on the right. Take the o

The solution is hidden so you can try first

10 min read · 10 sections

Python code

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

PythonFixed window: largest sum of k consecutive values, and the work it saves

Practice

PythonVariable windows: shortest subarray with sum at least target, longest ones with k flips