Subarray in middle
Input
find_subarray_with_sum([1, 2, 3, 7, 5], 12)
Output
(1, 3)
The subarray [2, 3, 7] (indices 1 to 3) sums to 12.
Full lesson preview
Find a subarray with a given sum (positive numbers)
Locate a contiguous subarray of positive integers that sums to a target using the sliding window technique.
Python practice17 minArrays – Fundamentals & PatternsIntermediateLast updated March 25, 2026
Problem statement
Given a list of positive integers and a target sum, find any contiguous subarray (by indices) that sums exactly to the target. If such a subarray exists, return a tuple (start_index, end_index) with 0-based inclusive indices. If no such subarray exists, return None. Because all numbers are positive, you can use a sliding window approach for O(n) time.
Task
Return the (start_index, end_index) of any contiguous subarray whose sum equals the target, or None if none exists. Use O(n) time with a sliding window.
Examples
Input format
Two arguments: a list of positive integers and an integer target, e.g. ([1,2,3,7,5], 12).
Output format
Return a tuple (start_index, end_index) of 0-based inclusive indices, e.g. (1, 3), or None if not found.
Constraints
All numbers are positive. 0 <= len(arr) <= 10^5. 1 <= target <= 10^9. Use O(n) time, O(1) extra space.
Samples
Sample 1
Input
[1, 4, 20, 3, 10, 5], 33
Output
(2, 4)
20 + 3 + 10 = 33, indices 2 through 4.