How Do I Choose Between a Hash Table and a Trie (Prefix Tree)?

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Advantages of tries:

The basics:

  • Predictable O(k) lookup time where k is the size of the key
  • Lookup can take less than k time if it’s not there
  • Supports ordered traversal
  • No need for a hash function
  • Deletion is straightforward

New operations:

  • You can quickly look up prefixes of keys, enumerate all entries with a given prefix, etc.

Advantages of linked structure:

  • If there are many common prefixes, the space they require is shared.
  • Immutable tries can share structure. Instead of updating a trie in place, you can build a new one that’s different only along one branch, elsewhere pointing into the old trie. This can be useful for concurrency, multiple simultaneous versions of a table, etc.
  • An immutable trie is compressible. That is, it can share structure on the suffixes as well, by hash-consing.

Advantages of hashtables:

  • Everyone knows hashtables, right? Your system will already have a nice well-optimized implementation, faster than tries for most purposes.
  • Your keys need not have any special structure.
  • More space-efficient than the obvious linked trie structure (see comments below)

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