Longest Substring with At Least K Repeating Characters

/*
Logic:
Test case: "bbaaacbd"  3
All the characters whose count is less than k can't be inside our answer so whenever we
get those characters,there is two possible answer string before that character and after that
character and pass those strings into the same function.
eg. "bbaaacbd"  k=3
     "bbaaa"  "bd"
     "" "aaa"  ""
     answer="aaa"
*/
class Solution {
public:
    int longestSubstring(string s, int k) {
        int n=s.length();
        if(n==0 || n<k) return 0;   //string size 0 or k>size of string,answer is 0
        if(k<=1) return n;          // if k<=1,then whole string will be our answer.

        unordered_map<char,int> mp;
        for(auto itr:s) mp[itr]++;

        int l=0;
        while(l<n && mp[s[l]]>=k) l++;   //lth character will be a character which shouldn't
        if(l>= n-1) return l;            // be in the answer,so break the string.

        int ans1=longestSubstring(s.substr(0,l),k);
        while(l<n && mp[s[l]]<k) l++;  //remove all those characters whose frequency is
        int ans2=0;                    // less than k.
        if(l<n) ans2=longestSubstring(s.substr(l),k);
        return max(ans1,ans2);
    }
};