Sliding Window: Turn Repeated Range Work Into Linear Time | NextGen CTO

Back to All Blogs1 min read10 Sept 2026

DSA Patterns That Actually Matter · Part 2 of 5

Sliding Window: Turn Repeated Range Work Into Linear Time

“Learn fixed and variable windows for subarrays and substrings.”

Sliding Window

Sliding window is useful for contiguous subarrays or substrings.

function maxWindowSum(nums: number[], k: number) {
  let sum = 0;

  for (let i = 0; i < k; i++) {
    sum += nums[i];
  }

  let best = sum;

  for (let right = k; right < nums.length; right++) {
    sum += nums[right];
    sum -= nums[right - k];

    best = Math.max(best, sum);
  }

  return best;
}

Variable Window

Expand right
     ↓
Window becomes valid
     ↓
Shrink left while possible
     ↓
Save answer

Common examples include longest substring without repeating characters and minimum-size subarray.

Note

Not every subarray problem is a sliding-window problem.

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