Longest Substring with At Least K Repeating Characters

https://www.lintcode.com/problem/longest-substring-with-at-least-k-repeating-characters/description

public class Solution {
    private int max = 0;
    /**
     * @param s: a string
     * @param k: an integer
     * @return: return an integer
     */
    public int longestSubstring(String s, int k) {
        // write your code here
        recursion(s, k);
        return max;
    }

    private void recursion(String s, int k) {
        int[] target = new int[26];
        char[] chars = s.toCharArray();
        for (char c : chars) {
            target[c - 'a']++;
        }
        int lastIndex = 0;
        for (int i = 0; i <= chars.length; i++) {
            if (i == chars.length) {
                if (lastIndex != 0) {
                    recursion(s.substring(lastIndex, i), k);
                    lastIndex = i + 1;
                }
                break;
            }
            char aChar = chars[i];
            if (target[aChar - 'a'] < k) {
                recursion(s.substring(lastIndex, i), k);
                lastIndex = i + 1;
            }
        }
        if (lastIndex == 0) {
            max = Math.max(max, s.length());
        }
    }
}