5 Sliding Window Variations You Will See in Interviews

Fixed window length

You get an array and a length k, and look for the best subarray of exactly k elements.

"Given an array visitors of daily website visits, return the highest total over any 3 consecutive days."

↓k = 3

↓

Best 3-day total = 26

Sum limit

You get a limit k and want the longest subarray whose sum stays within it:

- below k - exactly k - at most k

"Given an array minutes and a budget k, return the most consecutive tasks you can finish within k minutes."

↓k = 9

↓

Longest length = 4

Count limit

You want the longest stretch that contains at most k of something, such as zeros, vowels or one specific character.

"Return the longest run of 1s you can get by flipping at most one 0."

↓k = 1

↓

Longest length = 6

No repeats

You want the longest stretch in which every element is different.

"Given a string s, return the length of its longest substring with no repeated letters."

↓s = "abcabd"

↓

Longest length = 4

Counting windows

Instead of the best window, you count how many windows satisfy the constraint.

"Given positive integers nums and k, count the subarrays whose product is less than k."

↓k = 50

↓

When the window ends at index r, it adds r - l + 1 new subarrays.

Total = 1 + 2 + 2 + 3 = 8