Encode and Decode Strings

Encode and Decode Strings

What is this problem about?

The Encode and Decode Strings coding problem asks you to design an algorithm to serialize a list of strings into a single string and then deserialize that single string back into the original list. This is a common problem in communication protocols where you need to send multiple data items over a single stream. The challenge is handling strings that contain any possible character, including delimiters you might want to use.

Why is this asked in interviews?

This is a high-signal question used by companies like Microsoft and Meta to test a candidate's understanding of data serialization and state management. It evaluates how you handle edge cases, such as empty strings, strings containing special characters, or strings that look like your encoding format. The Encode and Decode Strings interview question reveals whether a candidate can design a robust protocol.

Algorithmic pattern used

The most effective design interview pattern for this is Chunked Encoding (Length-Prefixing).

1. Encode: For each string, prepend its length and a separator (e.g., #) to the string itself. Example: ["hello", "world"] becomes 5#hello5#world.
2. Decode: Read the encoded string. Find the first # to get the length L. Read the next L characters as the string. Repeat until the end of the input. This approach is robust because the length prefix tells you exactly how many characters to read, regardless of the content of the string.

Example explanation

Input: ["4#", "abc"]

1. Encode:
   • "4#" has length 2. Encoded: 2#4#.
   • "abc" has length 3. Encoded: 3#abc.
   • Full string: 2#4#3#abc.
2. Decode:
   • Find #: it's at index 1. Length is 2.
   • Read 2 chars after index 1: 4#.
   • Next # is at index 5. Length is 3.
   • Read 3 chars: abc. Result: ["4#", "abc"].

Common mistakes candidates make

• Using a single delimiter: Simply joining with a comma (,). This fails if one of the strings itself contains a comma.
• Escaping complexity: Trying to use escape characters (like ``) which makes the decoding logic much harder to implement correctly.
• Not handling empty strings: Failing to correctly encode ["", ""].

Interview preparation tip

When designing an encoding format, "Length + Delimiter" is almost always better than "Delimiter + Escaping." It is easier to implement, faster to decode (no need to scan for escapes), and inherently handles all character types.