题目链接：https://leetcode.com/problems/longest-substring-with-at-least-k-repeating-characters/#/description
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.
思路：先遍历整个string，并记录每个不同的character的出现次数。如果所有character出现次数都不小于k，那么说明整个string就是满足条件的longest substring，返回原string的长度即可；如果有character的出现次数小于k，假设这个character是c，因为满足条件的substring永远不会包含c，所以满足条件的substring一定是在以c为分割参考下的某个substring中。所以我们需要做的就是把c当做是split的参考，在得到的String[]中再次递归，找到最大的返回值即可。
class Solution{ public: int longestSubstring(string s,int k) { vector<int> bucket(26,0); for(auto c:s) bucket[c-'a']++; char delim=NULL; for(int i=0;i<26;i++) { if(bucket[i]>0 && bucket[i]<k) { delim=(char)(i+'a'); } } if(delim==NULL) return (int)s.length(); vector<string> res; int maxlen=0; stringstream ss(s); string g; while(getline(ss,g,delim)) { res.push_back(g); } for(auto str:res) { maxlen=max(maxlen,longestSubstring(str,k)); } return maxlen; } };
