Counting Subarrays!

• Time : $O(n)$
• Space : $O(1)$

int Solution::solve(vector<int> &A, int B) {
    int res = 0, sum = 0, l = 0;
    for(int i = 0; i < A.size(); i++) {
        sum += A[i];
        while(l < i and sum >= B) sum -= A[l++];
        if(sum < B) {
            res += i - l + 1;
        }   
    }  
    return res;
}