395

class Solution {
public:

    int getMaxUniqueLetters(string s){
        bool map[26] = {0};
        int unique = 0;
        for(char c: s){
            if(!map[c-'a']){
                unique++;
                map[c-'a'] = true;
            }
        }
        return unique;
    }

    int longestSubstring(string s, int k) {
        int count[26] = {0};
        int maxUnique = getMaxUniqueLetters(s);
        int ans = 0;
        for(int currUnique=1; currUnique<=maxUnique; currUnique++){
            memset(count, 0, sizeof(count));
            int left=0, right=0, unique=0, countAtLeastK=0;
            while(right<s.length()){
                if(unique <= currUnique){
                    int c = s[right]-'a';
                    if(count[c] == 0) unique++;
                    count[c]++;
                    if(count[c] == k) countAtLeastK++;
                    right++;
                }
                else{
                    int c = s[left]-'a';
                    if(count[c] == k) countAtLeastK--;
                    count[c]--;
                    if(count[c] == 0) unique--;
                    left++;
                }
                if(unique == currUnique && countAtLeastK == currUnique)
                    ans = max(ans, right-left);
            }
        }
        return ans;
    }
};