340. Longest Substring with At Most K Distinct Characters

class Solution {
public:
    int lengthOfLongestSubstringKDistinct(string s, int k) {
        vector<int> cnt(256, 0);
        int dis = 0, idx = 0, res = 0;
        if (!k) return 0;
        for (int i = 0; i < s.size(); i++) {
            if (!cnt[s[i]]) {
                while (dis >= k && idx < i) {
                    cnt[s[idx]]--;
                    dis -= cnt[s[idx++]] == 0;
                }
                dis++;
            }
            cnt[s[i]]++;
            res = max(res, i - idx + 1);
        }
        return res;
    }
};