A simple variation to "Longest Substring with At Most Two Distinct Characters". A typical sliding window problem.

class Solution {public:    int lengthOfLongestSubstringKDistinct(string s, int k) {        unordered_map
     
       hm;                int ret = 0, start = 0;        for (int i = 0; i < s.length(); i++)        {            hm[s[i]]++;            if (hm.size() <= k)            {                ret = std::max(ret, i - start + 1);            }            else            {                while (start < i && hm.size() > k)                {                    char nc = s[start];                    if (hm[nc] == 1)    hm.erase(nc);                    else                hm[nc]--;                                        start++;                }            }        }        return ret;    }};