int longestSubstringKDistinct(string str, int k){
    int start = 0, res = 0;
    unordered_map<char, int> um;
    for (int i = 0; i < str.size(); ++i)
    {
        um[str[i]]++;
        while(um.size() > k){
            char left = str[start];
            if(--um[left] == 0)
                um.erase(left);
            start++;
        }
        res = max(res, i - start + 1); //no need to put in while block as if the size is k, the res will be updated
    }
    return res;
}