题目：
题解：
class Solution {
public:int longestSubstring(string s, int k) {int ret = 0;int n = s.length();for (int t = 1; t <= 26; t++) {int l = 0, r = 0;vector<int> cnt(26, 0);int tot = 0;int less = 0;while (r < n) {cnt[s[r] - 'a']++;if (cnt[s[r] - 'a'] == 1) {tot++;less++;}if (cnt[s[r] - 'a'] == k) {less--;}while (tot > t) {cnt[s[l] - 'a']--;if (cnt[s[l] - 'a'] == k - 1) {less++;}if (cnt[s[l] - 'a'] == 0) {tot--;less--;}l++;}if (less == 0) {ret = max(ret, r - l + 1);}r++;}}return ret;}
};
