Serialize and deserialize a dictionary trie
Company: Snowflake
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: Medium
Interview Round: Onsite
##### Question
Design and implement serialization and deserialization for a **trie** (prefix tree) that stores a large dictionary of lowercase English words. The trie supports shared prefixes, arbitrary branching, and per-node end-of-word marking. Your design must produce a compact, parseable wire format and be able to faithfully reconstruct the original trie.
Implement the two operations:
```
serialize(root) -> bytes / string
deserialize(data) -> root
```
Address the following in your design and discussion:
1. **Wire format.** Specify a compact encoding that captures the trie structure: which characters branch from each node, the shared-prefix sharing inherent to the trie, and an explicit **end-of-word marker** on nodes that terminate a word.
2. **Traversal order.** Compare a **DFS (pre-order)** encoding against a **BFS (level-order)** encoding. Discuss the trade-offs for compactness, streaming, and reconstruction.
3. **Framing.** Compare a **delimiter-based** scheme against a **length-prefix** scheme for delimiting children/nodes. Discuss **escape-character handling** if delimiters can collide with payload bytes.
4. **Robustness.** Detect and handle **malformed or partial/truncated input** gracefully (raise a clear error rather than producing a corrupt trie or crashing).
5. **Streaming.** Make the encoding **streaming-friendly** so a producer can write and a consumer can read incrementally without buffering the whole structure in memory.
6. **Evolvability.** Provide **forward/backward compatibility** so future fields (e.g. word frequency, payloads) can be added without breaking old readers/writers.
7. **Extensions.** Discuss handling **Unicode** (beyond a–z), **optional compression**, and behavior under tight **memory limits**.
8. **Complexity.** Analyze the **time and space complexity** of both serialize and deserialize.
Provide pseudocode for both `serialize` and `deserialize`.
Quick Answer: Snowflake software-engineer onsite coding question: design a compact, robust wire format to serialize and deserialize a dictionary trie. Covers DFS vs BFS encodings, delimiter vs length-prefix framing and escaping, explicit end-of-word markers, streaming, malformed/partial-input handling, forward/backward compatibility, Unicode, compression, memory limits, and O(N) complexity.
A **trie** (prefix tree) stores a dictionary of words. Each node carries an end-of-word flag and a map of labeled child edges (one character per edge), so words sharing a prefix share the path to their divergence point.
Design a wire format and implement `serialize(root) -> string` and `deserialize(data) -> root` so the trie can be faithfully reconstructed.
**Recommended format (DFS pre-order, length-prefixed):** emit each node as `<eow_flag '0'/'1'>` then `<num_children>` (decimal digits terminated by `.`), then for each child in sorted order its edge character followed by that child's subtree. Because every node announces its child count up front, decoding is a single self-describing pass with **no delimiters and no escaping** — which also makes it streaming-friendly and robust to truncation (a bounds-checked reader raises a clear error instead of building a corrupt trie).
**Your task (to make this runnable & checkable):** implement a function `solution(words)` that builds a trie from the input list `words`, serializes it, deserializes the result back into a fresh trie, and returns the **sorted list of distinct words** reconstructed from the rebuilt trie. A correct serialize/deserialize round-trip must return exactly the set of input words (deduplicated, sorted), including the empty string if present.
**Example:** `words = ["cat", "car", "card", "dog"]` returns `["car", "card", "cat", "dog"]`.
**Follow-ups to discuss:** DFS vs BFS encoding; delimiter vs length-prefix framing (and escape handling); detecting malformed/partial input; forward/backward compatibility via a magic+version header; Unicode (UTF-8 edges), optional compression, and bounded-memory streaming.
Constraints
- 0 <= len(words) <= 10^4
- Each word consists of lowercase English letters 'a'-'z' (the empty string "" is allowed and marks the root as end-of-word).
- 0 <= len(word) <= 100
- Duplicate words may appear in the input; the reconstructed trie stores each distinct word once.
- The returned list must be sorted in ascending (lexicographic) order.
Examples
Input: (["cat", "car", "card", "dog"],)
Expected Output: ['car', 'card', 'cat', 'dog']
Explanation: Shared prefixes 'ca' and 'car' are stored once in the trie; the round trip recovers all four words sorted.
Input: ([],)
Expected Output: []
Explanation: Empty dictionary: the trie is just a non-terminal root with zero children; serialize/deserialize round-trips to an empty word set.
Input: (["a"],)
Expected Output: ['a']
Explanation: Single one-character word: root has one child 'a' marked end-of-word.
Input: (["", "a", "ab"],)
Expected Output: ['', 'a', 'ab']
Explanation: The empty string marks the root itself as end-of-word; the format's per-node eow flag must preserve this on the root.
Input: (["zebra", "zebra", "ze"],)
Expected Output: ['ze', 'zebra']
Explanation: Duplicate 'zebra' is stored once; 'ze' is an internal node that is also a terminal word along the path to 'zebra'.
Input: (["banana", "band", "ban", "can"],)
Expected Output: ['ban', 'banana', 'band', 'can']
Explanation: Branching at 'ban' (to 'banana' and 'band') and a separate root branch 'can' exercise arbitrary branching and shared prefixes.
Hints
- Model the trie as a labeled n-ary tree: each node has an end-of-word boolean and a map from edge character to child node.
- For a self-describing format, write each node's child count BEFORE its children. The decoder then reads exactly that many (character, subtree) pairs — no delimiters, no escaping.
- Pre-order DFS pairs naturally with recursion: emit the current node, then recurse into each child. Deserialize is the mirror image driven by the counts in the stream.
- Make every read bounds-checked. If the stream ends mid-node, a flag is out of range, or there are trailing bytes after the root subtree, raise a clear error rather than returning a half-built trie.
- To verify a round trip, collect words by DFS from both the original and the rebuilt trie; the deduplicated, sorted lists must match.