Hi, this side Imsfg. I used both map/unordered_map since long time. I observed some facts about about this special stl.

1- According to me use mp[i]==0 instead of mp.find(i) because let suppose you increase the value of mp[i]++.after that you decrease the same value mp[i]--. after u find mp.find(i). it show true but according to you this give 0 and does not exist in map. if you not understand what i am saying please see both code u can observed what i say. Case 1-use mp[i]==0 int longestKSubstr(string s, int k) { int i=0,j=0; int cnt=0; int ans=-1; map<char,int>mp; while(i<s.size()){ while(cnt>k and j<=i){ mp[s[j]]--; if(mp[s[j]]==0){ cnt--; } j++; } if(mp[s[i]]==0){ cnt++; } if(cnt==k){ ans=max(ans,i-j+1); } mp[s[i]]++; i++; }

return ans;
}

Case 2- use mp.find(i)

    //User function template for C++

class Solution{ public: int longestKSubstr(string s, int k) { setst; int i=0,j=0; int cnt=0; int ans=-1; map<char,int>mp; while(i<s.size()){ while(cnt>k and j<=i){ mp[s[j]]--; if(mp[s[j]]==0){ cnt--; } j++; } if(mp.find(s[i])==mp.end()){ cnt++; } if(cnt==k){ ans=max(ans,i-j+1); } mp[s[i]]++; i++; }

return ans;
}

};

hopefully You understand what I am saying.....