How do you serialize and deserialize a binary tree?

Question presented to candidate: "Design two Python functions: serialize, which turns a binary tree into a string, and deserialize, which turns that string back into an identical tree. The format is up to you, but the round trip must be exact for any tree."

What a strong answer should cover:

• Why a plain traversal is not enough: different shapes can share the same preorder, so missing children must be recorded with a null marker.
• Preorder with null markers: serialize recursively; deserialize by consuming tokens in the same order with one moving position. O(n) time and space.
• The level-order (BFS) alternative used by LeetCode: a queue, null for gaps, trailing nulls trimmed; deserialize by reading two tokens per parent.
• A delimiter between tokens so negative and multi-digit values parse correctly.
• Trade-offs: string size of each format, recursion depth versus an iterative queue.
• Python specifics: build a list and ",".join it, consume tokens with iter() + next(), use collections.deque (not list.pop(0)) for BFS, and remember recursion depth on deep trees.

Clarifying questions expected:

• What values can nodes hold (negative numbers, very large numbers, strings)?
• Does the format need to be human-readable or compatible with anything else?

Code / implementation expected: Yes — preorder serialize / deserialize (and optionally the BFS version), in Python 3.