Serialize a List of Arbitrary Strings into One String and Back, Then Compare Schemes

Two components exchange data over a channel that carries exactly one string per message. You need to send a whole list of strings through it: the sender converts the list into one string, and the receiver must rebuild exactly the same list from that string alone. Implement both sides: ```python def encode(strs: list[str]) -> str: """Convert a list of strings into a single string.""" def decode(s: str) -> list[str]: """Rebuild the original list from a string produced by encode.""" ``` `decode(encode(strs)) == strs` must hold for every possible list. After the implementation, the interviewer discusses the advantages and disadvantages of different encoding and decoding approaches. ### Constraints and Clarifications - A string may contain any character, including whatever separator, digit or escape character you might pick. A format that only works when some character never appears in the data does not meet the requirement. - Strings may be empty, and so may the list. The empty list and a list containing one empty string are different inputs and must decode to different results. - Order must be preserved, and duplicate strings are allowed. - `decode` receives nothing except the encoded string: no extra arguments and no state shared with `encode`. - Design the format yourself. Do not call a general-purpose serialization library such as `json` or `pickle`, and do not evaluate the encoded string as code. Those libraries may still come up in Part 2. ### Clarifying Questions - Are the values Unicode text or raw bytes, and does the channel carry arbitrary bytes or only printable text? - Is there an upper bound on the length of one string or on the size of the whole list? - Does the receiver always get the complete message, or must it start decoding while data is still arriving? - Will code written in other languages ever produce or consume this format? ### Part 1 — Implement encode and decode Write `encode` and `decode` so that every list round-trips exactly under the rules above. Walk through how your decoder handles a string that contains the character your format treats as special, and state the time and space complexity of both functions. ```hint Separators can be data Any character you reserve can also occur inside a string. Ask what the decoder could learn about the next string before reading it, or how the encoder could mark which occurrences of a special character are real boundaries. ``` #### What This Part Should Cover - A format that is unambiguous for every input, including special characters inside the data, empty strings and the empty list - A decoder that follows the format exactly and rejects malformed input instead of guessing - Linear-time encoding and decoding, including how the output string is built - Test cases that would expose a broken format ### Part 2 — Compare encoding schemes Describe at least two schemes that differ from the one you implemented, and compare them with yours. Which would you choose for this channel, and what would make you change that choice? ```hint Axes of comparison Compare the schemes on concrete costs: extra characters per string, typically and in the worst case; how much of the data the decoder must inspect; whether either side can start before it has seen everything; and what a single corrupted character does to the rest of the message. ``` #### What This Part Should Cover - At least two genuinely different schemes, and how each one keeps decoding unambiguous - Space overhead and decoding cost, typical and worst case - Behavior with streaming input, very long strings, and corrupted or truncated messages - A justified choice tied to the properties of the channel ### What a Strong Answer Covers - A correctness argument that holds for adversarial input, not just typical strings - The empty list kept distinct from a list of one empty string - A format specified precisely enough that someone else could write a compatible decoder - Honest complexity analysis, including the cost of building the output - Trade-offs connected to the answers to the clarifying questions: text versus bytes, streaming, and interoperability ### Follow-up Questions - If the strings are Unicode text and the channel carries bytes, should a length count characters or encoded bytes, and what breaks when the sender and receiver disagree? - How would you change the format if the sender receives each string piece by piece and does not know its length until it ends? - How would you detect a truncated or corrupted message instead of silently returning a wrong list? - How would you version the format so that an older decoder cleanly rejects data from a newer encoder?