https://leetcode.com/problems/longest-substring-with-at-least-k-repeating-characters/description/

题目描述

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.

解决思路：子串肯定是连续的，要求满足要求的最长子串，那么找到所有子串，然后找最大的就是了，那些不符合的要求的字母，会把源字符串分割成一个一个子串，在这些子串上在用同样的方法求解。

 int longestSubstring(string s, int k) {
     int len = s.size();
     vector<int> cnt(26, 0);
     for(int i = 0; i<len; i++){
         cnt[s[i]-'a']++;
     }


     char res = 'a'-1;
     for(int i = 0; i<cnt.size(); i++){
         if(cnt[i]>0 && cnt[i]<k){
             res='a'+i;
         }
     }

     if(res =='a'-1) return len;

     int idx = s.find(res, 0);
     int left = longestSubstring(s.substr(0, idx), k);
     int right = longestSubstring(s.substr(idx+1, len-idx-1), k);
     return max(left, right);
 }