Count distinct elements in every window of size k

vector <int> countDistinct (int A[], int n, int k)
{
    unordered_map<int, int> um;
    vector <int> result;
	// count number of distinct elements for first window of size k
    for (int i = 0; i < k; i++) 
        um[A[i]]++;
    
    result.push_back(um.size());

	// calculate answer for rest of the windows
    for (int i = 1; i < n - k + 1; i++) {
        um[A[i - 1]]--;
        if (um[A[i - 1]] <= 0) 
            um.erase(A[i - 1]);
        
        um[A[i + k - 1]]++;

        result.push_back(um.size());
    }
    return result;
}