题目：

题解：

func longestSubstring(s string, k int) (ans int) {for t := 1; t <= 26; t++ {cnt := [26]int{}total := 0lessK := 0l := 0for r, ch := range s {ch -= 'a'if cnt[ch] == 0 {total++lessK++}cnt[ch]++if cnt[ch] == k {lessK--}for total > t {ch := s[l] - 'a'if cnt[ch] == k {lessK++}cnt[ch]--if cnt[ch] == 0 {total--lessK--}l++}if lessK == 0 {ans = max(ans, r-l+1)}}}return ans
}func max(a, b int) int {if a > b {return a}return b
}