Binary Subarrays With Sum.cpp

class Solution {
public:
       int numSubarraysWithSum(vector<int>& A, int S) {
        unordered_map<int, int> c({{0, 1}});
        int psum = 0, res = 0;
        for (int i : A) {
            psum += i;
            res += c[psum - S];
            c[psum]++;
        }
        return res;
    }
};