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.

intuition: use a non-ASCII character as a delimiter to join the strings, such as Character.toString((char)257)

encode: time = O(n), space = O(1)

decode: time = O(n), space = O(n)

public class Codec {

    // Encodes a list of strings to a single string.
    public String encode(List<String> strs) {
        if(strs.size() == 0) {
            return Character.toString((char)258);
        }
        
        String d = Character.toString((char)257);
        StringBuilder sb = new StringBuilder();
        for(String str : strs) {
            sb.append(str);
            sb.append(d);
        }
        sb.deleteCharAt(sb.length() - 1);
        return sb.toString();
    }

    // Decodes a single string to a list of strings.
    public List<String> decode(String s) {
        String d = Character.toString((char)258);
        if(s.equals(d)) {
            return new ArrayList<>();   // empty string
        }
        
        d = Character.toString((char)257);
        // split(String regex, int limit), limit controls the number of times the pattern is applied and therefore affects the length of the resulting array. 
        // If the limit n is greater than zero then the pattern will be applied at most n - 1 times, the array's length will be no greater than n, and the array's last entry will contain all input beyond the last matched delimiter. 
        // If n is non-positive then the pattern will be applied as many times as possible and the array can have any length. 
        // If n is zero then the pattern will be applied as many times as possible, the array can have any length, and trailing empty strings will be discarded.
        return Arrays.asList(s.split(d, -1));
    }
}

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