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;
}

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

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.

思路：把字串陣列按照長度+內容的格式進行編解碼。

public class Codec {

    // Encodes a list of strings to a single string.
    public String encode(List<String> strs) {
        char[] lenstr = new char[4];
        StringBuilder sb = new StringBuilder();
        for(String str: strs) {
            int len = str.length();
            for(int j=3; j>=0; j--) {
                lenstr[j] = (char)(len & 0b11111111);
                len >>= 8;
            }
            sb.append(new String(lenstr));
            sb.append(str);
        }
        return sb.toString();
    }

    // Decodes a single string to a list of strings.
    public List<String> decode(String s) {
        List<String> decoded = new ArrayList<>();
        int pos = 0;
        while (pos < s.length()) {
            int len = 0;
            for(int i=0; i<4; i++) len = (len<<8) + s.charAt(pos++);
            decoded.add(s.substring(pos, pos+len));
            pos += len;
        }
        return decoded;
    }
}

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