amazon k string find k-1 character

vector<string> findK_1(string s, int k) {
    unordered_map<char, int> charMap;
    int dup = 0;
    int start = 0;
    unordered_set<string> strSet;
    vector<string> res;
    
    for (int i = 0; i < s.size(); i++) {
        charMap[s[i]]++;
        if (charMap[s[i]] == 2) {
            dup++;
        }
        if (charMap[s[i]] == 3) {
            while (start <= i) {
                charMap[start]--;
                if (charMap[start] == 2) {
                    start++;
                    break;
                }
                start++;
            }
        }
        if (dup == 2) {
            while (start <= i) {
                charMap[start]--;
                if (charMap[start] == 1) {
                    start++;
                    dup--;
                    break;
                }
                start++;
            }
        }
        if (i - start + 1 == k) {
            if (dup == 1) {
                strSet.insert(s.substr(start, k));
            }
            charMap[start]--;
            if (charMap[start] == 1) {
                dup--;
            }
            start++;
        }
       
    }
    
    res = new vector<string>(strSet.begin(), strSet.end());
    return res;
}