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.

这个题蛮力法肯定不行，思路是扫描出现次数小于k的字母，然后将它们作为分隔符，然后就将这些子串划分成原问题的子问题，递归求解。

public class Solution {
public int longestSubstring(String s, int k) {
if(k<=1){
return s.length();
}

int[] repeat=new int[26];
for(int i=0;i<s.length();i++){
repeat[s.charAt(i)-'a']++;
}
StringBuilder reg=new StringBuilder("");
boolean firstSplit=true;
for(int i=0;i<26;i++){
if(repeat[i]>0&&repeat[i]<k){
if(firstSplit){
reg.append((char)(i+'a'));
firstSplit=false;
}
else{
reg.append("|"+(char)(i+'a'));
}
}
}
if(reg.length()>0){
//说明有分隔符
String[] strs=s.split(reg.toString());
int max=0;
int tmpAns=0;
for(String str:strs){
tmpAns=longestSubstring(str, k);
if(tmpAns>max){
max=tmpAns;
}
}
return max;
}
else{
//没有分隔符,说明s中的每一个字符出现的次数都大于等于k
return s.length();
}
}
}