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Trie vs Hash Map for String Problems
Quick Answer
Use a Trie when you need prefix-based operations: autocomplete, prefix search, longest common prefix, or when keys share structure and you want to avoid storing full strings. Use a Hash Map when you only need exact lookups (is this string in the set?) and prefix queries are not required.
Side-by-Side Comparison
| Aspect | Trie | Hash Map |
|---|---|---|
| Lookup type | Prefix, exact, and range by prefix | Exact only |
| Structure | Tree of characters; shared prefixes overlap | Flat key-value pairs |
| Space | O(n × avg key length); shared prefixes save space | O(n × avg key length); each key stored fully |
| Insert / lookup | O(m) per operation, m = key length | O(m) average; hash + compare |
| Prefix search | Native: traverse to prefix node, DFS for all suffixes | Must iterate all keys and filter by prefix O(n) |
| When optimal | Autocomplete, prefix matching, word search, IP routing | Exact membership, anagrams, frequency by exact key |
| Ordered iteration | DFS yields lexicographic order naturally | No inherent order; sort if needed |
Decision Framework
- Do you need "all strings with prefix X" or autocomplete? Use Trie.
- Do you only need "is string S in the set"? Hash map is simpler.
- Are keys highly overlapping (e.g., "cat", "car", "card")? Trie saves space.
- Do you need longest common prefix or prefix-based routing? Trie.
- Is it a word search or dictionary problem with prefix constraints? Trie.
- Do you need to count frequencies of exact strings? Hash map.
Example Problems
Trie
- 1.Implement Trie — Insert, search, startsWith in O(m) per operation.
- 2.Design Add and Search Words — Trie + DFS for wildcard "." matching.
- 3.Word Search II — Build trie from words; DFS grid, prune when no prefix match.
- 4.Longest Common Prefix — Trie: insert all, traverse until branch; or binary search.
Hash Map
- 1.Valid Anagram — Count chars; no prefix needed, exact comparison.
- 2.Group Anagrams — Map sorted string → list; exact key lookup.
- 3.First Unique Character — Count frequencies; no prefix structure.
- 4.Ransom Note — Count magazine chars; check if note chars exist. Exact lookup.