Count number of substrings.cpp

class Solution
{
  public:
    long long int AtMostK (string s, int k)
    {
        if (k < 0) return 0;
    
        int i = 0, j = 0, cnt = 0;
        long long int res = 0;
        int m[26] = {0};
    
        while (j < s.length ())
        {
            m[s[j] - 'a']++;
            if (m[s[j] - 'a'] == 1) cnt++;
    
            while (cnt > k)
            {
                m[s[i] - 'a']--;
                if (m[s[i] - 'a'] == 0) cnt--;
    
                i++;
            }
    
            res += (j - i + 1);
            j++;
        }
        return res;
    }

    long long int substrCount (string s, int k) {
        return AtMostK (s, k) - AtMostK (s, k - 1);
    }
};