```
public class Solution {
public int longestSubstring(String s, int k) {
if (s == null || s.length() == 0) return 0;
char[] chars = new char[26];
// record the frequency of each character
for (int i = 0; i < s.length(); i += 1) chars[s.charAt(i) - 'a'] += 1;
boolean flag = true;
for (int i = 0; i < chars.length; i += 1) {
if (chars[i] < k && chars[i] > 0) flag = false;
}
// return the length of string if this string is a valid string
if (flag == true) return s.length();
int result = 0;
int start = 0, cur = 0;
// otherwise we use all the infrequent elements as splits
while (cur < s.length()) {
if (chars[s.charAt(cur) - 'a'] < k) {
result = Math.max(result, longestSubstring(s.substring(start, cur), k));
start = cur + 1;
}
cur++;
}
result = Math.max(result, longestSubstring(s.substring(start), k));
return result;
}
}
```

In each step, just find the infrequent elements (show less than k times) as splits since any of these infrequent elements couldn't be any part of the substring we want.