930

class Solution {
public:
    int atMost(vector<int> & A, int S){
        int ans, left, right;
        ans = 0;
        if(S < 0) return 0;
        for(left=right=0; right<A.size(); right++){
            S -= A[right];
            while(S < 0)
                S += A[left++];
            ans += right-left+1;
        }
        return ans;
    }
    int numSubarraysWithSum(vector<int>& A, int S) {
        return atMost(A, S) - atMost(A, S-1);
    }
};