Count distinct elements in every window

class Solution{
  public:
    vector <int> countDistinct (int A[], int n, int k)
    {
        vector<int> ans;
        unordered_map<int, int> m;
        for(int i=0; i<k; i++)
        {
           m[A[i]]++;
        }
        
        int left = 0;
        int cnt = m.size();
        ans.push_back(cnt);
        
        if(n>k)
        {
            for(int i=k; i<n; i++)
            {   //add this element into map
                m[A[i]]++;
                if(m[A[i]]==1)
                    cnt++;
                    
                 //discard the last element   
                m[A[left]]--;
                if(m[A[left]] == 0)
                    cnt--;
                left++;
                
                ans.push_back(cnt);
            }
        }
        
        return ans;
    }
};