Xai·Software Engineer·Technical Phone Screen·Senior

Senior

Jul 2026

Summary

xAI software engineer technical screen, one coding problem the whole time. Classic phone keypad encoding thing, looked familiar but the trie angle tripped me up a bit.

Questions Asked (1)

Given a dictionary of words and a list of digit-string queries, return all dictionary words whose phone keypad encoding starts with each query, sorted lexicographically. Each digit 2-9 maps to a group of letters the way old T9 phones worked.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

My first instinct was to just encode every word upfront and then for each query do a prefix scan.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem constraints (dictionary size, query count, word lengths) and then propose a trie-based solution where each node represents a digit. Preprocess the dictionary by encoding each word into its digit sequence and inserting it into a trie, then for each query traverse the trie to collect all words under the prefix, sorting them lexicographically. Discuss trade-offs between preprocessing time/space and query efficiency, and consider alternatives like sorting the encoded words for binary search.

Pro tip: Mention that you can store words at each trie node in sorted order during insertion to avoid sorting at query time, and highlight that this approach is efficient for multiple queries. Also, proactively discuss how to handle edge cases like empty queries or no matches.

1. Clarify requirements and constraints

Ask about dictionary size, number of queries, maximum word length, and whether queries can be empty or contain digits other than 2-9. Confirm that the output should be sorted lexicographically for each query.

2. Design the data structure

Propose building a trie where each node corresponds to a digit (2-9) and stores a list of words that pass through it. Alternatively, consider encoding all words to digit strings and sorting them for binary search.

3. Preprocess the dictionary

For each word, convert it to its digit sequence using the keypad mapping, then insert it into the trie, appending the word to the list at each node along the path. Keep the lists sorted to avoid post-processing.

4. Process each query

Traverse the trie according to the query digits. If the traversal fails, return an empty list. Otherwise, collect all words stored at the final node (which are already sorted) as the result.

5. Analyze complexity and trade-offs

Discuss time and space complexity: preprocessing O(N*L) where N is number of words and L is average length, query time O(Q*L + total output size). Compare with alternative approaches like sorting encoded words and using binary search, highlighting pros and cons.

Key Points to Mention

Trie data structure with digit-based nodes
Preprocessing dictionary by encoding words to digits
Sorting words at each trie node during insertion to avoid sorting at query time
Time complexity: O(N*L) preprocessing, O(Q*L + output) per query
Space complexity: O(N*L) for trie storage
Handling edge cases: empty queries, no matches, and non-digit characters
Trade-offs between trie and sorting-based approaches

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.