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Google·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
May 2026Remote

Summary

Google SWE interview with a pattern matching design problem that's nastier than it looks on the surface. The core ask was building a rule engine, and if you haven't thought carefully about wildcard semantics before, you'll probably underestimate it like I did.

Questions Asked (1)

Q1

Design a rule engine that takes a binary string as input and matches it against a list of (pattern, value) pairs, where each pattern can contain '0', '1', and '*' (where '*' matches any substring including empty). The match function should return the value of the longest matching pattern, with ties broken by list order.

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

I went straight to regex and said we could just compile each pattern and run them all.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then propose a trie-based solution that handles wildcards efficiently. Explain how to traverse the trie while tracking the longest match and tie-breaking by list order. Discuss trade-offs between preprocessing time, query time, and memory usage.

Pro tip: Mention that you can preprocess patterns into a trie with wildcard transitions, and during matching, use BFS/DFS to explore all possible paths, keeping track of the best match. This shows you understand both algorithmic efficiency and practical implementation details.

1. Clarify Requirements

Ask about input size, pattern length, number of patterns, and expected query frequency to determine if preprocessing is beneficial.

2. Choose Data Structure

Propose a trie (prefix tree) where each node represents a character, and wildcard '*' is handled as a special transition that can match any substring.

3. Design Matching Algorithm

Traverse the trie with the input string, exploring all possible paths when encountering '*', and track the longest match and its value, using list order for ties.

4. Analyze Complexity

Discuss time and space complexity: preprocessing O(total pattern length), query time potentially exponential in worst case but optimizable with memoization or dynamic programming.

5. Discuss Trade-offs and Optimizations

Compare with brute-force matching, mention memoization to avoid redundant states, and consider alternative approaches like regex or dynamic programming.

Key Points to Mention

  • Trie data structure for efficient prefix matching
  • Handling wildcard '*' as a transition that can match any substring, including empty
  • Tracking the longest match and tie-breaking by list order
  • Time and space complexity analysis, including worst-case scenarios
  • Memoization or dynamic programming to optimize wildcard matching
  • Trade-offs between preprocessing and query time, and memory usage

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