271. Encode and Decode Strings

class Codec {
public:

    // Encodes a list of strings to a single string.
    string encode(vector<string>& strs) {
        string res;
        for (int i = 0; i < strs.size(); i++)
            res += to_string(strs[i].size()) + "%" + strs[i];
        return res;
    }

    // Decodes a single string to a list of strings.
    vector<string> decode(string s) {
        vector<string> res;
        for (int i = 0; i < s.size(); ) {
            int len = 0;
            while (isdigit(s[i])) len = len * 10 + s[i++] - '0';
            i++;
            string tmp;
            while (len--) tmp += s[i++];
            res.push_back(tmp);
        }
        return res;
    }
};

// Your Codec object will be instantiated and called as such:
// Codec codec;
// codec.decode(codec.encode(strs));