BinarySubarraysWithSum(prefix sum).py

class Solution:
    def numSubarraysWithSum(self, A: List[int], S: int) -> int:
        '''
        prefix sum
        '''
        P = [0]
        for x in A:
            P.append(P[-1] + x)
        count = collections.Counter()

        ans = 0
        for x in P:
            ans += count[x]
            count[x + S] += 1
        return ans