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.

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/longest-substring-with-at-least-k-repeating-characters
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分治算法，每次统计所有字符，找到不够数的字母，把字符串分成两边，再对两边计算。

class Solution {
    public int longestSubstring(String s, int k) {
        char[] counts = new char[26];
        for (int i = 0; i < s.length(); i++) {
            counts[s.charAt(i) - 'a'] ++;
        }

        int target = -1;
        for (int i = 0; i < s.length(); i++) {
            if (counts[s.charAt(i) - 'a'] < k) {
                target = i;
                break;
            }
        }
        if (target == -1) {
            return s.length();
        }
        return Math.max(longestSubstring(s.substring(0, target), k), longestSubstring(s.substring(target + 1), k));
    }
}