LeetCode 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.

Runtime: 3 ms

public int longestSubstring(String s, int k) {
if (s == null || s.length() == 0) return 0;
int[] nums = new int[26];
char[] chars = s.toCharArray();
for (char c : chars) nums[c - 'a']++;
boolean valid = false, flag = false;
for (int num : nums) {
if (num >= k) valid = true;
if (num > 0 && num < k) flag = true;
if (valid && flag) break;
}
if (!valid) return 0;// invalid: no char in s appears >= k times.
if (!flag) return s.length();//!flag: every char appears >= k times.
int max = 0, start = 0, cur = 0;
for (int i = 0; i < s.length(); i++) {
if (nums[chars[i] - 'a'] < k) {
max = Math.max(max, longestSubstring(s.substring(start, i), k));
start = i + 1;
}
}
max = Math.max(max, longestSubstring(s.substring(start), k));
return max;
}