LeetCode 930. Binary Subarrays With Sum

In an array A of 0s and 1s, how many non-empty subarrays have sum S?

Example 1:

Input: A = [1,0,1,0,1], S = 2
Output: 4
Explanation: 
The 4 subarrays are bolded below:
[1,0,1,0,1]
[1,0,1,0,1]
[1,0,1,0,1]
[1,0,1,0,1]

Note:

1. A.length <= 30000
2. 0 <= S <= A.length
3. A[i] is either 0 or 1.

Solution: Prefix Sum

counts[s] := # of subarrays start from 0 that have sum of s
ans += counts[s – S] if s >= S

Time complexity: O(n)
Space complexity: O(n)