Explain Trie vs HashMap for Prefix Search.
A trie (prefix tree) efficiently supports prefix-based lookups, while a hash map offers fast exact-key access but lacks native prefix search capabilities.
When to Use
Use a trie when building autocomplete, spell-check, or search-as-you-type systems that need prefix lookups. Use a hashmap when you only need exact key-value retrieval and don’t care about partial matches.
Example
Typing “int” should suggest “interview” and “internet.” A trie traverses nodes for each character to find matches quickly, whereas a hashmap would need to scan every key.
To master such trade-offs and design efficient systems, explore:
- Grokking System Design Fundamentals
- Grokking the System Design Interview
- Grokking the Coding Interview
- Mock Interviews with ex-FAANG engineers
Why Is It Important
Prefix search underpins user-facing features like search suggestions and auto-completion. Choosing the right data structure impacts both latency and memory footprint in production systems.
Interview Tips
Explain that tries perform prefix lookups in O(p) (p = prefix length), while hashmaps give O(1) only for exact matches. Demonstrate with an autocomplete example and discuss scalability.
Trade-offs
Trie
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Pros: Fast prefix lookup and ordered traversal
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Cons: High memory usage
HashMap
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Pros: O(1) lookups and low memory
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Cons: Cannot do efficient prefix search.
Pitfalls
A common mistake is assuming hashmaps can handle prefixes efficiently. They can’t without full scans. Tries, while powerful, can consume more memory due to large branching.
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