DSA Pattern: Sliding Window

Sliding window is a pattern for problems about continuous ranges inside an array or string.

Instead of recomputing a range again and again, the window moves one step at a time while reusing previous work.

Fixed-size window

Use this when the problem gives a size k.

Example: maximum sum of any subarray of length k.

function maxSumOfK(values, k) {
	let windowSum = 0;

	for (let i = 0; i < k; i++) {
		windowSum += values[i];
	}

	let best = windowSum;

	for (let right = k; right < values.length; right++) {
		windowSum += values[right];
		windowSum -= values[right - k];
		best = Math.max(best, windowSum);
	}

	return best;
}

Variable-size window

Use this when the window expands and shrinks based on a condition.

Common examples include longest substring without repeating characters and smallest subarray with sum at least target.

Complexity

Most sliding window solutions are O(n) because each pointer moves forward at most n times.

Common signs

• subarray or substring
• contiguous range
• longest or shortest range
• fixed length k
• at most or at least condition

Practice problems

1. Maximum sum of a subarray of size k.
2. Average of every subarray of size k.
3. Longest substring without repeating characters.
4. Smallest subarray with sum at least target.
5. Longest subarray with at most two distinct values.

Practice links

• LeetCode Sliding Window problems
• NeetCode practice roadmap