395. Longest Substring with At Least K Repeating Characters

Find the length of the longest substring T of a given string (consists of lowercase letters only) such that every character in T appears no less than k times.

Example 1:

Input:
s = "aaabb", k = 3

Output:
3

The longest substring is "aaa", as 'a' is repeated 3 times.

Example 2:

Input:
s = "ababbc", k = 2

Output:
5

The longest substring is "ababb", as 'a' is repeated 2 times and 'b' is repeated 3 times.

class Solution {
    public int longestSubstring(String s, int k) {
        int res = 0;
        for(int i = 0; i <= s.length() - k; i++){
            for(int j = i + k; j <= s.length(); j++){
                String t = s.substring(i, j);
                if(help(t, k)){
                     res = Math.max(res, j - i);
                }
            }
        }
        return res;
    }
    public boolean help(String s, int k){
        int[] arr = new int[128];
        for(char c: s.toCharArray()) arr[c]++;
        for(int i: arr){
            if(i != 0 && i < k) return false;
        }
        return true;
    }
}

1. brute force, TLE

class Solution {
    public int longestSubstring(String s, int k) {
        if (s == null || s.length() == 0) return 0;
        if (k<2) return s.length();
        return helper(s, 0, s.length(), k);
    }

    public int helper(String s, int l, int r, int k) {
        if (l>=r) return 0;
        
        // build freq map
        int[] freq = new int[26];
        for (int i=l; i<r; i++) freq[s.charAt(i)-'a']++;
        
        // check if valid
        boolean valid = true;
        for (int i=0; i<26 && valid; i++) if (freq[i] > 0 && freq[i] < k) valid = false;
        if (valid) return r-l;
        
        // if not for each invalid character start a new split search
        int best = 0, start=l;
        for (int i=l; i<r; i++) {
            if (freq[s.charAt(i) -'a'] < k) {
                best = Math.max(best, helper(s, start, i, k));
                start = i+1;
            }
        }
        best = Math.max(best, helper(s, start, r, k));
        return best;
    }
}

https://leetcode.com/problems/longest-substring-with-at-least-k-repeating-characters/discuss/87738/Java-20-lines-very-easy-solution-7ms-with-explanation