Encode and decode a list of strings

Design an encoder that packs a list of strings (which may contain any character, including colons, hashes, newlines, or empty strings) into one contiguous string, and a decoder that reconstructs the exact original list.

Before you code

If you join strings with a delimiter like , or #, what happens when one of the strings itself contains that character? Instead of searching for a sentinel at the end of each string, store the byte length of the string right in front of it, followed by a single separator such as #. Once the decoder reads the length k before #, it jumps forward k characters in constant time without scanning for delimiters inside the payload.

Hints

1. Format each string s as "{len(s)}#{s}" — for example, "code" becomes "4#code" and "" becomes "0#".
2. In the decoder, keep a cursor i at the start of the current record. Scan forward until # to parse the integer length k.
3. Slice the next k characters starting right after #, append that substring to your result list, and advance i to hash_index + 1 + k.

Production & complexity notes

When solving Encode and decode a list of strings on LeetCode (medium), always state your loop or recursion invariant before writing code. Verify how your chosen data structure behaves on degenerate inputs (single-element inputs, all-duplicate keys, strictly monotonic sequences, or maximum constraint sizes up to $10^5$ elements). In production interviews, pair your implementation with an explicit time and auxiliary space complexity proof, explain why alternative brute-force approaches violate the latency budget, and trace how cache locality and memory allocation overhead affect wall-clock throughput on real hardware. Review the step-by-step memory trace in Contiguous and scattered data to reinforce the underlying pattern.

What it practices

Flattening scattered pointer-based strings into a single contiguous length-prefixed buffer, from Contiguous and scattered data.