Distinct Numbers in Window

• Time : $O(n)$
• Space : $O(n)$

vector<int> Solution::dNums(vector<int> &A, int B) {
    vector<int> res;
    int now = 0;
    unordered_map<int, int> freq;
    for(int i = 0; i < A.size(); i++) {
        if(++freq[A[i]] == 1) now += 1;
        if(i >= B) {
            if(--freq[A[i-B]] == 0) now -= 1;
        }
        if(i >= B - 1) res.push_back(now);
    }
    return res;
}