Nice logic. I modified your logic a bit. First deleted all the repeated elements and them made the comparison. C++ code

int alpha[26] = {0};
int cnt = 0;
vector <string> str(N);
for(int i = 0; i < N; i++)
{
    cin >> str[i];
    sort(str[i].begin(), str[i].end());
    str[i].erase(unique(str[i].begin(), str[i].end()), str[i].end());

    for(int k = 0; k < str[i].size(); k++)
    {
        char c = str[i].at(k);
        alpha[c - 'a']++;
        if(alpha[c - 'a'] == N)
            cnt++;
    }
}