424. Longest Repeating Character Replacement

class Solution {public:    int characterReplacement(string s, int k) {        int best = 0;        int l = 0;        unordered_map<char, int> mapping;        char maxc = ' ';        int maxcount = 0;        for (int r = 0; r < s.size(); r++) {            // going to add char to window first            mapping[s[r]] += 1;            // look through mapping for most common char so far            if (mapping[s[r]] >= maxcount) {                maxc = s[r];                maxcount = mapping[s[r]];            }            // cout << l << " " << r << " " << maxc << " " << maxcount <<            // endl;            // now fix the sliding window by decreasing l until its valid            while ((r - l + 1) - maxcount > k) {                // cout << (r - l + 1) - maxcount << endl;                // cout << l << " " << r << endl;                mapping[s[l]]--;                // if (s[l] == maxc){                //     maxcount--;                // }                l++;            }            // then update best;            best = max(best, r - l + 1);        }        return best;    }};