Solution Explanation: Encode and Decode Strings

This problem requires designing an algorithm to encode a list of strings into a single string and then decode it back to the original list. The solution avoids using serialization methods like eval.

Approach:

The core idea is to prepend each string with its length. This allows us to reconstruct the original strings during decoding. We'll use a delimiter or encoding scheme to separate the length information from the string itself.

Encoding:

1. Length Encoding: For each string in the input list, we determine its length. We can represent this length using a fixed number of digits (e.g., 4 digits) to ensure consistent parsing during decoding. This makes it easy to extract the length from the encoded string.

2. Concatenation: We concatenate the length (as a string) and the original string. This creates a single encoded string.

Decoding:

1. Length Extraction: We iterate through the encoded string. At each iteration, we extract the length from the first few digits (four in our case).

2. String Extraction: Using the extracted length, we can accurately extract the corresponding string from the encoded string.

3. List Construction: We append each extracted string to a list, which will be our decoded list of strings.

Time and Space Complexity:

• Time Complexity: Both encoding and decoding have a linear time complexity, O(N), where N is the total number of characters in all strings (or total length of the encoded string). This is because we iterate through the strings (or encoded string) once.

• Space Complexity: The space complexity is also O(N) for both encoding and decoding because the encoded string and the decoded list have a size proportional to the total length of the original strings.

Code Examples (Python, Java, C++, Go):

The code examples below demonstrate this approach. Each example shows both encode and decode functions. Note that the specific implementation details might vary slightly between languages, primarily in how string manipulation and length encoding are handled.

Python3:

class Codec:
    def encode(self, strs: List[str]) -> str:
        ans = []
        for s in strs:
            ans.append(f"{len(s):04d}{s}") # f-string for formatted length
        return "".join(ans)
 
    def decode(self, s: str) -> List[str]:
        ans = []
        i = 0
        while i < len(s):
            length = int(s[i:i+4])
            ans.append(s[i+4:i+4+length])
            i += 4 + length
        return ans

Java:

public class Codec {
    public String encode(List<String> strs) {
        StringBuilder sb = new StringBuilder();
        for (String str : strs) {
            sb.append(String.format("%04d", str.length())).append(str);
        }
        return sb.toString();
    }
 
    public List<String> decode(String s) {
        List<String> res = new ArrayList<>();
        int i = 0;
        while (i < s.length()) {
            int len = Integer.parseInt(s.substring(i, i + 4));
            res.add(s.substring(i + 4, i + 4 + len));
            i += 4 + len;
        }
        return res;
    }
}

C++:

class Codec {
public:
    string encode(vector<string>& strs) {
        string encoded = "";
        for (const string& str : strs) {
            encoded += to_string(str.length());
            encoded += string(4 - to_string(str.length()).length(), '0'); // Padding with zeros
            encoded += str;
        }
        return encoded;
    }
 
    vector<string> decode(string s) {
        vector<string> decoded;
        int i = 0;
        while (i < s.length()) {
            int len = stoi(s.substr(i, 4));
            decoded.push_back(s.substr(i + 4, len));
            i += 4 + len;
        }
        return decoded;
    }
};

Go:

type Codec struct{}
 
func (c *Codec) Encode(strs []string) string {
    encoded := ""
    for _, str := range strs {
        encoded += fmt.Sprintf("%04d", len(str)) + str
    }
    return encoded
}
 
func (c *Codec) Decode(s string) []string {
    decoded := []string{}
    i := 0
    for i < len(s) {
        lenStr := s[i : i+4]
        length, _ := strconv.Atoi(lenStr)
        decoded = append(decoded, s[i+4 : i+4+length])
        i += 4 + length
    }
    return decoded
}

These examples all follow the same fundamental approach but adapt the syntax and string manipulation techniques to their respective programming languages. Remember to handle potential errors (e.g., invalid input format) in a production-ready implementation.