Leetcode: Binary Subarrays With Sum

Binary Subarrays With Sum

Similar Problems:

• Subarray Product Less Than K
• Tag: #subarray, #presum, #inspiring, #classic

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.

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

• Solution:

// Blog link: https://code.dennyzhang.com/binary-subarrays-with-sum
// Basic Ideas: Presum + Dynamic Programming
//     Count how many subarray ends with current item
// Complexity: Time O(n), Space (n)
func numSubarraysWithSum(A []int, S int) int {
    res, preSum := 0, 0
    h := map[int]int{0: 1}
    for _, v := range A {
       preSum += v
       res += h[preSum - S]
       h[preSum]++
    }
    return res
}