Subarray with B odd numbers

• Time : $O(n)$
• Space : $O(n)$

int Solution::solve(vector<int> &A, int B) {
    if(B == 0) {
        int now = 0;
        int res = 0;
        for(auto& a : A) {
            if(a & 1) now = 0;
            else now += 1;
            res += now;
        }
        return res;
    }
    vector<int> st{-1};
    for(int i = 0; i < A.size(); i++) if(A[i] & 1) st.push_back(i);
    st.push_back(A.size());
    int res = 0;
    for(int i = B; i < st.size() - 1; i++) {
        int l = st[i - B + 1] - st[i - B];
        int r = st[i + 1] - st[i];
        res += l * r;
    }
    return res;
}