Count number of Substrings which has exactly k distinct characters

https://practice.geeksforgeeks.org/contest/interview-series-45/problems/#

Edutorial :  
1) Hashmap + Sliding Window  -->> https://iq.opengenus.org/substrings-with-k-distinct-characters/
2) Freq[26] + Sliding Window  -->> youtube.com/watch?v=rTefnFlCwpI
 

#include<bits/stdc++.h>
using namespace std;


class Solution
{
  public:
    long long int helper(string s, int k){
        // unordered_map<char,long long int> freq;
        vector<int> freq(26, 0);
    	
    	long long int n = s.size();
    	long long int j = 0;// pointer to the starting of sliding window
    	long long int cnt1 = 0;//count of substring with atmost k distinct characters
    	long long int distinct = 0;
    	for(long long int i=0;i<n; i++){
    	    freq[s[i] - 'a']++;
    	    if(freq[s[i] - 'a'] == 1)distinct++;
    	    
    	    
    	    while(distinct > k and j <= i){// decrease window size by deleting elements 
    	    // from the start of the window -> to make atmost k distict characters
    	    
    	       freq[s[j]- 'a']--;
    	       if(freq[s[j] - 'a'] == 0)distinct--;
    	       
    	       j++;
    	        
    	    }
    	    cnt1 += i - j + 1;
    	}
    	
    	return cnt1;
        
    }
    long long int substrCount (string s, int k) {
    	// number of substrings with exactly k distinct characters = 
    	// number of substrings with atmost k distinct characters - number of substrings
    	// with atmost k - 1 distinct characters
    	
    	
    	// How to calculate the number of strings with atmost k distinct characters?
    	// -> Hashmap (to check distinctness) + Sliding window  -->> TLE(it should work as well)
    	// -> freq[26] + Sliding window  -->> AC
    	
    	
    	return helper(s , k ) - helper(s, k -1);
    	
    	
    	
    }
};



int main()
{
    int t; cin >> t;
    while (t--)
	{
		string s; cin >> s;
		int k; cin >> k;
		Solution ob;
		cout <<ob.substrCount (s, k) << endl;
	}