271. Encode and Decode Strings

Design an algorithm to encode a list of strings to a string. The encoded string is then sent over the network and is decoded back to the original list of strings.

Machine 1 (sender) has the function:

string encode(vector<string> strs) {
  // ... your code
  return encoded_string;
}

Machine 2 (receiver) has the function:

vector<string> decode(string s) {
  //... your code
  return strs;
}

So Machine 1 does:

string encoded_string = encode(strs);

and Machine 2 does:

vector<string> strs2 = decode(encoded_string);

strs2 in Machine 2 should be the same as strs in Machine 1.

Implement the encode and decode methods.

Note:

• The string may contain any possible characters out of 256 valid ascii characters. Your algorithm should be generalized enough to work on any possible characters.

• Do not use class member/global/static variables to store states. Your encode and decode algorithms should be stateless.

• Do not rely on any library method such as eval or serialize methods. You should implement your own encode/decode algorithm.

class Codec {
public:

    // Encodes a list of strings to a single string.
    string encode(vector<string>& strs) {
        string encode_str = "";
        for (const string& str : strs) {
            encode_str += to_string(str.length()) + "@" + str;
        }
        return encode_str;
    }

    // Decodes a single string to a list of strings.
    vector<string> decode(string s) {
        vector<string> res;
        int pos = 0;
        while (pos < s.length()) {
            int at_pos = s.find_first_of('@', pos);
            int len = stoi(s.substr(pos, at_pos - pos));
            pos = at_pos + 1;
            res.push_back(s.substr(pos, len));
            pos += len;
        }
        return res;
    }
};