# Cracking the coding interview: Subarray with given sum — 6

Given an unsorted array A of size N that contains only non-negative integers, find a continuous sub-array which adds to a given number S.

In case of multiple subarrays, return the subarray which comes first on moving from left to right.

Example 1:

`Input:N = 5, S = 12A[] = {1,2,3,7,5}Output: 2 4Explanation: The sum of elements from 2nd position to 4th position is 12.`

Example 2:

`Input:N = 10, S = 15A[] = {1,2,3,4,5,6,7,8,9,10}Output: 1 5Explanation: The sum of elements from 1st position to 5th positionis 15.`

You don’t need to read input or print anything. The task is to complete the function subarraySum() which takes arr, N and S as input parameters and returns an arraylist containing the starting and ending positions of the first such occurring subarray from the left where sum equals to S. The two indexes in the array should be according to 1-based indexing. If no such subarray is found, return an array consisting only one element that is -1.

Expected Time Complexity: O(N)
Expected Auxiliary Space: O(1)

Constraints:
1 <= N <= 105
1 <= Ai <= 109

Solution:

Time complexity: O(n)

Space complexity: O(1)

For this question, in the for loop, both variables, num and sum, are created. num is used to store the number of values being added in array list, and sum means the total value of the values being from the list. When sum has not yet reached the objective value of s and num does not exceed the maximum number of values, then keep adding the index value from the list. Until it reaches sum is equivalent to s, it could eventually add both the beginning index position and ending position into arr2, then return it. Else, return -1.