题目：

题解：

int longestSubstring(char* s, int k) {
    int ret = 0;
    int n = strlen(s);
    for (int t = 1; t <= 26; t++) {
        int l = 0, r = 0;
        int cnt[26];
        memset(cnt, 0, sizeof(cnt));
        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 = fmax(ret, r - l + 1);
            }
            r++;
        }
    }
    return ret;
}