// intialize frequency container
void countFreq(int a[], int n, unordered_map<int, int> &hm) {
    // Insert elements and their
    // frequencies in hash map.
    for (int i = 0; i < n; i++)
        hm[a[i]]++;
}//O(n)

// return frequency number
int freqQuery(int x, unordered_map<int, int> hm) {
    return hm[x];
}//O(1)


int main() {
    int a[] = {1, 3, 2, 4, 2, 1};
    unordered_map<int, int> hm;  //create frequency container
    countFreq(a, 6, hm);         // intialize frequency container

    cout << freqQuery(2, hm) << endl;  // 2
    cout << freqQuery(3, hm) << endl;  // 1
    cout << freqQuery(5, hm) << endl;  // 0
}