395. longest substring with at least k repeating characters

public int (String s, int k) {
    if (s == null || s.length() == 0 || k < 1) {
        return s.length();
    }
    return f(s, 0, s.length(), k);
}
public int f(String s, int a, int b, int k) {
    if (b < a) return 0;
    if (b - a + 1 < k) return 0;
    //记录下来每一个字符出现的频率
    int[] arr = new int[26];
    for (int i = a; i < b; i++) {
        arr[s.charAt(i) - 'a']++;
    }
    int len = b - a;
    for (int i = a; i < b; i++) {
        //找到小于k次的字符，然后用它分割整个区间
        if (arr[s.charAt(i) - 'a'] < k && arr[s.charAt(i) - 'a'] > 0) {
            //二分法求值
            int left = f(s, i + 1, b, k);
            int right = f(s, a, i, k);
            return Math.max(right, left);
        }
    }
    return len;
}