482. 密钥格式化

链接：482. 密钥格式化

题解：

class Solution {
public:
    string licenseKeyFormatting(string s, int k) {
        if (s.size() <= 0) {
            return s;
        }
        std::string result;
        //result.reserve(s.size());
        int tail = 0;
        int k_cnt = 0;
        for (int i = s.size()-1; i >= 0; --i) {
            if (s[i] == '-') {
                continue;
            }
            if (k_cnt >= k) {
                result.push_back('-');
                ++i;
                k_cnt = 0;
                continue;
            }
            if (s[i] >= '0' && s[i] <= '9') {
                result.push_back(s[i]);
            } else if (islower(s[i])) {
                result.push_back(toupper(s[i]));
            } else {
                result.push_back(s[i]);
            }
            ++k_cnt;
        }
        reverse(result.begin(), result.end());
        return result;
    }
};