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Google·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

Google SWE interview that went into some interesting string processing territory. The follow-up question is the part that stuck with me.

Questions Asked (1)

Q1

Given a string that may have been 'jammed' by a keyboard (causing repeated characters), determine which dictionary words could have produced it. Then optimize so you're not scanning the entire dictionary on every query. For example, can you tell immediately that 'banana' could never match 'hheelllo'?

Algorithms & Data StructuresSystem DesignTechnical Trade-offs
Author's notes

The base version I got through fine.

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AI HintsAI Generated

Suggested Approach

First, clarify the problem: a jammed string is formed by repeating each character of the original word one or more times. Then, design a data structure that indexes dictionary words by their run-length encoded signature (character and count) to enable fast queries. For each query, compute its signature and use the index to retrieve candidate words, verifying matches efficiently.

Pro tip: Emphasize that the run-length encoding of the query must exactly match the run-length encoding of the word in terms of character sequence, but the counts in the query must be greater than or equal to the counts in the word. This insight allows you to quickly eliminate impossible matches like 'banana' vs 'hheelllo' because the character sequences differ.

1. Clarify the problem and constraints

Confirm that the jammed string is produced by repeating each character of the original word one or more times, and that the dictionary is static. Discuss potential constraints like dictionary size, query frequency, and memory limits.

2. Design the matching condition

Define that a word matches a query if their run-length encoded sequences have the same characters in the same order, and for each character, the query's count is >= the word's count. This condition is necessary and sufficient.

3. Preprocess the dictionary

Compute the run-length encoding for each dictionary word and build an index keyed by the character sequence (e.g., 'helo' for 'hello'). Store the count vectors for each word under its key.

4. Optimize query processing

For a query, compute its run-length encoding. Use the character sequence as a key to retrieve candidate words from the index. For each candidate, check if the query's counts are >= the word's counts. Return all matches.

5. Analyze trade-offs and edge cases

Discuss time and space complexity: preprocessing O(total characters in dictionary), query O(length of query + number of candidates). Consider edge cases like empty strings, single-character words, and queries with no matches.

Key Points to Mention

  • Run-length encoding (RLE) as the core representation for both dictionary words and queries.
  • The necessary and sufficient condition for a match: same character sequence and query counts >= word counts.
  • Indexing by the character sequence (e.g., using a hash map) to avoid scanning the entire dictionary.
  • Handling queries with characters not present in any dictionary word (e.g., 'banana' vs 'hheelllo' fails immediately due to different character sequences).
  • Complexity analysis: preprocessing time and space, query time proportional to query length plus number of candidates.
  • Potential optimizations: storing counts as arrays, using tries for prefix matching, or filtering by length constraints.

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