Strong Password Checker — C++

// Time:  O(n)
// Space: O(1)

class Solution {
public:
    int strongPasswordChecker(string s) {
        int missing_type_cnt = 3;
        missing_type_cnt -= static_cast<int>(any_of(s.begin(), s.end(), [](char c){ return isdigit(c); }));
        missing_type_cnt -= static_cast<int>(any_of(s.begin(), s.end(), [](char c){ return isupper(c); }));
        missing_type_cnt -= static_cast<int>(any_of(s.begin(), s.end(), [](char c){ return islower(c); }));

        int total_change_cnt = 0;
        int one_change_cnt = 0, two_change_cnt = 0, three_change_cnt = 0;
        for (int i = 2; i < s.length();) {
            if (s[i] == s[i - 1] && s[i - 1] == s[i - 2]) {
                int length = 2;
                while (i < s.length() && s[i] == s[i - 1]) {
                    ++length;
                    ++i;
                }
                total_change_cnt += length / 3;
                if (length % 3 == 0) {
                    ++one_change_cnt;
                } else if (length % 3 == 1) {
                    ++two_change_cnt;
                } else {
                    ++three_change_cnt;
                }
            } else {
                ++i;
            }
        }

        if (s.length() < 6) {
            return max(missing_type_cnt, 6 - static_cast<int>(s.length()));
        } else if (s.length() <= 20) {
            return max(missing_type_cnt, total_change_cnt);
        }

        int delete_cnt = s.length() - 20;
        
        total_change_cnt -= min(delete_cnt, one_change_cnt) / 1;
        total_change_cnt -= min(max(delete_cnt - one_change_cnt, 0), two_change_cnt * 2) / 2;
        total_change_cnt -= min(max(delete_cnt - one_change_cnt - 2 * two_change_cnt, 0), three_change_cnt * 3) / 3;
            
        return delete_cnt + max(missing_type_cnt, total_change_cnt);
    }
};