DistinctLetters

int DistLetters(string s, int k)
{
    int longest = 0, start = 0, distinct_count = 0;
    array<int, 256> visited = {0};
    for (int i = 0; i < s.length(); ++i)
    {
        if (visited[s[i]] == 0)
            ++distinct_count;
        ++visited[s[i]];
        while(distinct_count > k)
        {
            --visited[s[start]];
            if (visited[s[start]] == 0)
            {
                --distinct_count;
            }
            ++start;
        }
        longest += i - start + 1;
    }
    return longest;

}
int DistinctLetters(string s, int L, int U)
{

    return (DistLetters(s,U)-DistLetters(s,L-1));
}