482. License Key Formatting

class Solution {
public:
    string licenseKeyFormatting(string S, int K) {
        string res;
        for (int i = S.size() - 1; i >= 0; i--)
            if (S[i] != '-') {
            if (res.size() % (K + 1) == K) res += '-';
            res += toupper(S[i]);
        }
        reverse(res.begin(), res.end());
        return res;
    }
};