It is very disappointed to see that useless blog gets upvotes and a lot of people comment on those blogs but I even did not got any answer of my genuine problem. I was thinking that there are so many genius coders that will answer my problem. Shame on you CodeForces community.

What's the need of it? You can write simple O(n^3) solution.

I don't think I can solve it

By quick thinking, $$$O(n^2 + alphabet)$$$ is possible if you iterating each letter and increase its frequency while maintaining the maximum and the minimum

int n = s.size();
long long res = 0;
for (int i = 0; i < n; ++i)
{
    for (int j = i; j < n; ++j)
    {
        ++cnt[s[j]];
        update(max); <-- you can just maximize(max, ++cnt[s[j]]); at the beginning
        update(min); <-- this can be a bit tricky to get O(1), but O(log alphabet) coding is simpler
        res += max - min;
    }

    for (int j = i; j < n; ++j)
        --cnt[s[j]];
}