Encode and Decode Strings

Problem Statement

The challenge involves creating a robust algorithm for encoding a list of strings into a single string format, which can then be transmitted over a network and subsequently decoded back into the original list of strings by another machine. This procedure is to be implemented through two functions:

1. encode(vector<string> strs) which is used by Machine 1 (sender) to transform the list of strings into an encoded string.
2. decode(string s) which allows Machine 2 (receiver) to interpret the encoded string back into the list of strings as originally sent.

The encoded format needs to be such that it can unambiguously differentiate between different strings in the list, even if they are complex or empty strings. The procedure should ensure that after decoding, Machine 2’s output list of strings (strs2) must be identical to the input list of strings (strs) used by Machine 1. The solution must avoid using any serialization methods like eval to meet the challenge requirements.

Examples

Example 1

Input:

dummy_input = ["Hello","World"]

Output:

["Hello","World"]

Explanation:

Machine 1:
Codec encoder = new Codec();
String msg = encoder.encode(strs);
Machine 1 ---msg---> Machine 2
Machine 2:
Codec decoder = new Codec();
String[] strs = decoder.decode(msg);

Example 2

Input:

dummy_input = [""]

Output:

[""]

Constraints

• 1 <= strs.length <= 200
• 0 <= strs[i].length <= 200
• strs[i] contains any possible characters out of 256 valid ASCII characters.

Approach and Intuition

Given the constraints and requirements, our solution needs to handle variable-length strings as well as a large range of potential character sets. Here's a step-by-step approach to tackle this:

1. Encoding Approach:

   • To uniquely identify where each string starts and ends in the encoded form, use a delimiter based approach that includes the length of each string.
   • For each string s in the list strs, encode it by prefixing with its length followed by a special character (for example, a hash #), and then the string itself. This will help in clearly demarking where the string begins and ends.
   • An example encoding might look like 5#Hello5#World for the input ["Hello", "World"].

2. Decoding Approach:

   • Parse the encoded string from beginning to end.
   • Read until you find a special delimiter (e.g., #), which indicates that the preceding number is the length of the string.
   • Knowing the length, you can then accurately capture the string of that length that follows.
   • Move the parsing cursor forward by the length of the string plus any additional characters used for encoding (like the length and delimiter), and then repeat the process until the entire encoded string is parsed.

This approach carefully handles the constraints:

• The delimiter and length prefix ensure that even strings that contain unusual or special characters (out of the 256 ASCII characters possible) or are empty are accurately encoded and decoded.
• Using the length of the string as part of the encoding ensures that the parsing during the decoding process is straightforward and unambiguous.

Solutions

class Codec {
public:
    string serialize(vector<string>& inputStrings) {
        string result;
        for (string &element : inputStrings) {
            result += to_string(element.size()) + "/:" + element;
        }
        return result;
    }
    
    vector<string> deserialize(string data) {
        vector<string> output;
        size_t startPos = 0;
        while (startPos < data.size()) {
            size_t delimiter = data.find("/:", startPos);
            int len = stoi(data.substr(startPos, delimiter - startPos));
            string extracted = data.substr(delimiter + 2, len);
            output.push_back(extracted);
            startPos = delimiter + 2 + len;
        }
        return output;
    }
};