340

class Solution {
public:
    int lengthOfLongestSubstringKDistinct(string s, int k) {
        unordered_map<char, int> cnt;
        int left=0, right=0, ans=0;

        for( ;right<s.length(); right++){
            if(cnt[s[right]]++ == 0) k--;
            while(left<=right && k<0){
                if(--cnt[s[left++]] == 0) k++;
            }
            ans = max(ans, right-left+1);
        }

        return ans;
    }
};