Basic example
Input
arr = [2, 1, 5, 1, 3, 2], K = 3
Output
9
Subarray [5,1,3] has the maximum sum 9.
Full lesson preview
Find the maximum sum of a subarray of size K
Compute the maximum sum among all contiguous subarrays of fixed size K using a sliding window.
Python practice16 minTwo Pointers & Sliding WindowIntermediateLast updated March 26, 2026
Problem statement
Given an array of integers arr and an integer K, return the maximum sum of any contiguous subarray of length K. If K is greater than the array length, return None.
Task
Use a variable update of the window sum to compute the maximum contiguous sum of size K in O(n) time.
Examples
Input format
An array of integers arr and an integer K.
Output format
An integer equal to the maximum sum, or None if K > len(arr).
Constraints
1 <= K <= 10^5, len(arr) <= 10^5. Integers may be negative. Time O(n) and O(1) extra space.
Samples
Sample 1
Input
arr = [2, 3, 4, 1, 5], K = 2
Output
7
Subarray [3,4] has sum 7.