leetcode LongestSubstringAtLeastKRepeating

public class LongestSubstringAtLeastKRepeating {
  public int longestSubstring(String s, int k) {
    /**
     * 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.
     */
    char[] as = s.toCharArray();
    return help(as, k, 0, s.length());
  }
  public int help(char[] a, int k, int start, int end){
    if (end - start < k) return 0;
    int[] count = new int[26];
    for (int i=start; i<end; i++){
      count[a[i]-'a'] += 1;
    }
    for (int i=0; i<count.length; i++){
      if (count[i]>0 && count[i]<k){
        for (int j=start; j<end; j++){
          if (a[j] == i+'a'){
            int left = help(a, k, start, j);
            int right = help(a, k, j+1, end);
            return Math.max(left, right);
          }
        }
      }
    }
    return end-start;
  }
}